Skip every 2nd File on New File

I have files currently named 0001, 0001_b, 0002, 0002_b, 0003, 0003_b

I need to be able to rename for example to

0031, 0031_b, 0032, 0032_b, 0033, 0033_b

I assume i will need a script, as New Name will just change to 0001, 0002_b, .. etc

I dont mind doing two passes to solve this, as in using a skip for all file names that have a _b in it, and then opposite to that in teh 2nd pass

Can anyone help with a script or idea for this

thanks so much

All you are doing is adding a fixed integer (30) to each number.

Renumber method...

Number position: 1
Change to: Relative to existing number
Number difference: 30
Zero padding: Automatic