Modules & VBA :: Renumber Records When One Number Changed?

I have a table which has 2 fields 1) Project_Priority_Number and 2) Previous_Priority_Number. If there were 100 records these would be numbered 1-100 in the order that the user originally sets the priority (this number is in addition to the record ID number). I have created a form with code that moves the Project_Priority_Number to the Previous_Priority_Number and then shows the Project_Priority_Number as blank and displays all of the records.

The user can then set new priority numbers in the blank column. Say they choose to make the old priority number 4 the new number 3 and priority 27 now becomes say 2, etc. I want the user to press a button that re-numbers the remaining ones based on their old position + or - 1 (basically to fill the gaps but based on their previous positions). I understand how to renumber if one is deleted but I don't know how to be more specific and re-number based both on their previous priority number and whether something else is now set to replace that.

For Example (the following numbers need to re-order based on previous priority 4 becomming the new priority 1 and the previous number 8 becomming 3 - so the previous 1 becomes 2 and then everything after the new 3 moves forward 1):

PROJECT PRIORITY NUMBER PREVIOUS PRIORITY NUMBER
1
2
3
1 4
5
6
7
3 8
9
10