Transition

Step 1 - Time "T" minus 1 Year
In order to allow for a long permissive period where both old and new style numbers can be dialed, I would add to the dialing pattern a special transition code "10". Dialing beginning 10+ would be followed by a 10D number. This could be promoted as "ten plus ten digits". This permissive dialing would last as long as 10+ dialing is not required for another use. Calls would simply go through during the permissive period. After the permissive period an announcement would give the new NPAX for that number. Giving the full number would be problematic as subscribers do change their numbers, so a more generic message would be given.

At the same time '001'+10D would be introduced as a replacement for 0+ dialing as part of the 7d -> 8d plan. The required timeout would be added to "00" operator access. "00"+ as "Sent Paid International" can also be introduced at this time. 
Step 1:
00   00                   00      LD Operator (timeout)
001  001  nxx nxx xxxx    001+    Operator Assisted NANP
00n  00 nxxxxxxxxxxxxxx   00+     Sent Paid International
10n  10   nxx nxx xxxx    10+10D  Permissive Sent Paid LD
                          "ten plus ten digits"
0+10D and 1+10D use remains during this permissive period.
7D and 10D remain where used.
NO changes to international inbound numbering
Step 2 - Time "T" minus 6 Months
At least 6 months after Step 1: 0+10D dialing will be removed from service as part of the progression toward 8D dialing. Callers will recieve a recording with the 001+ dialing instruction. 
Step 2:
0n   0    nxx nxx xxxx    0+10D    Removed from service
1+10D use remains during this permissive period.  7D and 10D remain where used.
NO changes to international inbound numbering
Step 3 - Time "T" Introduction of NPAX
At least 6 months after Step 2 (after 0+10D use has been sent to interupt and "cooled off") "0"+7D is permitted to numbers currently dialed 7D. At the same time, NPAX numbers are introduced as a whole. This combined move will take most subscribers from npa-nxx-xxxx numbers to their final npax-dnxx-xxxx numbers, with dnxx-xxxx being the 'local part' for 8D dialing. ('d' being '0' in non-overlay areas.)

Most 4D NPAs will be in the form of npa -> n9pa as previously discussed, but in overlay areas a common NPAX number will be assigned to all NPAs in that area. The next digit after the NPAX will be used to show which NPA the number originated in. Numbers outside of the US will also have special migration. As these are in easily defined areas they should be easier to explain to subscribers.

The NPA -> NPAX conversion will lead to a certain amount of confusion as people learn the new NPAX's. Calls will go through using "10"+10D which will help those without a lookup. As "10"+10D will eventually become a recording, subscribers should be cautioned from relying on the shortcut forever. This is a 'frustration relief' shortcut for those who just don't know the new NPAX codes and must get a call through.

In addition, if one calls n9pa to an npa that did not get that treatment will get an intercept directing the caller to the appropriate number. As a rule of thumb, all calls routed to an intercept should be non-chargeable or charged at a rate no higher than a full minute call to any other domestic number.

To support Canada's move to +1 00, the dialing code "100"+10D will be introduced. Officially, Canadian numbers will change from npa-nxx-xxxx to 00npa-nxx-xxxx from the NANP and international standpoint. Other nations will be moved to the +1 99 range, dialed using "199"+10D. Special consideration would need to be made to make sure that any existing 9xx NPAs in the US moving to 99xx do not conflict with 99-nxx areas in those countries. 
Step 3:
001  001 00 nxx nxx xxxx  001+00  Operator Assisted Canada
001  001  n9xx dnxx xxxx  001+    Operator Assisted NPAX
100  1 00   nxx nxx xxxx  100+    Canada
1n9  1    nxxx dnxx xxxx  1+      NPAX Numbers, includes
     1    n9xx 0nxx xxxx          Non-overlay areas
     1 99   nxx nxx xxxx          Non-US/Canada NANP
0n   0nxx xxxx            8D Dialing Permitted
n    dnxx xxxx            8D Dialing Permitted (Overlays)
     (Note: Careful selection of d is required.)
1+10D use remains during the permissive period.  
7D and 10D remain permissively where used.
What is dialed 8D is defined by the telephone companies and the state PUCs.
'Toll alerting' will be maintained in areas that wish to support it.
Changes to International Inbound Numbering
+1 00nxxnxxxxxx -> 00 nxx nxx xxxx Canada
+1 nxxnxxxxxx   ->    nxx nxx xxxx 10D Numbers (Permissive)
+1 n9xxxnxxxxxx ->  n9xx xnxx xxxx 12D NANP Numbers
+1 99nxxnxxxxxx -> 99 nxx nxx xxxx Non-US/Canada NANP
Basic Rule: if 2nd digit after +1 is not 9 or 0, it is a 10D number
Step 4 - Time "T" plus 6 Months/1 Year
A suitably LONG transition period is in effect where everyone reprints their stationary and tells people "my new number is ... npax-dnxx-xxxx" before the old system is peeled away. The transition period should not be long enough to allow subscribers to forget that the system IS changing, but should be long enough not to cause an immediate economic burden.

I expect the third transition period to be 6 months to one year.

At the end of the permissive period, the 'old' numbers are revoked and sent to interrupts. This includes all 7D dialing of numbers. 

Step 4:
001  001    npa nxx xxxx  001+    Intercept: Operator
10n  10     npa nxx xxxx  10+10D  Intercept: Redirect Recordings
1np  1      npa nxx xxxx  1+      Removed from service - use NPAX
                          (where p is not '9', all three cases)
n    nxx xxxx             Removed from service - use 8D
n    nxx nxx xxxx         Removed from service - use 8D
1+12D use remains, as well as 001+12D for Operator Assistance.
8D now in use per telephone company/state PUC decisions.
Changes to International Inbound Numbering
+1 00nxxnxxxxxx -> 00 nxx nxx xxxx Canada
+1 nxxnxxxxxx   ->    nxx nxx xxxx Removed from service
+1 n9xxxnxxxxxx ->  n9xx xnxx xxxx Required to new 12D numbers
+1 99nxxnxxxxxx -> 99 nxx nxx xxxx Non-US/Canada countries in NANP
Basic Rule: if 2nd digit after +1 is not 9 or 0, it is sent to intercept
Complete!
The move is now complete. The entire NANP has migrated to a new NUMBERING system.

A note on what is covered above. It is basically a US based description. There will be slight modifications in Canada and other NANP countries. It will be up to the regulators in the respective countries to rule, but the suggestion is:

  1. 0+7D becomes access to 7D locals, permissive with 7D where offered.
  2. 1+12D becomes 'entire NANP' dialing as with the US.
  3. 10+10D becomes/remains "Sent Paid" within the country.
  4. 001+12D becomes access to operator assisted calls.
  5. 10D numbers remain in overlay areas.
For more information on non-US NANP changes, please see the review.

[Introduction] - [Moving 7D -> 8D] - [12D Plan] - [Transition] - [Complete Review]