Update same field for other records based on edit to the field on one record.

Update same field for other records based on edit to the field on one record.

I have a form called Orders with 3 fields:
 
Project Number (Single Line), Customer Name (Single Line), Sequence (Number)
 
I need to set and change the order in which we will do each job regularly, hence the number field called Sequence that I sort records by. Example starting data:
 
100101      Good Customer      1
100105      Other Customer      2
100106      That Customer      3
100110      Excellent Customer      4
100112      Best Customer      5
 
I'm looking for a deluge code to attach to the Sequence field so that when I change a record, it updates all the other records. For example, if I wanted project/customer 100112 Best Customer to be Sequence 2, I need code that would run when I change Sequence from 5 to 2 for that record, finding all records that are already >= Sequence 2, and incrementing them by 1. Resulting in this:
 
100101      Good Customer      1
100112      Best Customer      2
100105      Other Customer      3
100106      That Customer      4
100110      Excellent Customer      5
 
Also, where is the best place to place this code? Obviously under Field Actions, and the Sequence field. But would On User Input or On Update be more appropriate?