We won't need to. The project prefix can be changed on the project, separate to the issues. The issue number field will actually contain a number, starting at 1, on a per project basis. This will then be converted to an issue number by padding the number (maybe to 6 figures), and then prepended the project issue prefix. For example:

Prefix: PROJ
Number: 10
Issue Number on output: PROJ000010

The reason we have to have a separate field to store the number is because we want the numbers to increment independently of other projects' issues.