Help to rename a list of file .tif

Hello
I've a list of about 38000 file .tif (scans of pictures) which are named in the following way:

Example, Directory nr. 126

126-0001.tif (the front of the picture)
126-0001a.tif (the rear of the same picture)

126-0002.tif
126-0002a.tif

126-0003.tif (no rear for this picture)

126-0004.tif

126-0005.tif
126-0005a.tif

126-0006.tif
126-0006a.tif
126-0006b.tif (more than a scan of the same picture)

I have to change the name of all these files starting from a new number (for example 35001) and I've to do this operation for each directory, starting from a new number.
The result of the example should be the following:

126-0001.tif 35001a.tif
126-0001a.tif 35001b.tif
126-0002.tif 35002a.tif
126-0002a.tif 35002b.tif
126-0003.tif 35003a.tif
126-0004.tif 35004a.tif
126-0005.tif 35005a.tif
126-0005a.tif 35005b.tif
126-0006.tif 35006a.tif
126-0006a.tif 35006b.tif
126-0006b.tif 35006c.tif

How can I do this operation? I've tried in different ways but with no success
Thank for your help
Roberto

You haven't completely specified your problem but as described you can solve it using two steps...

Working on one directory at a time:
1) Use the Replace method to replace the leading 3 digits plus "-0" with a 2-digit new prefix
2) Use List replace to replace the final letter

1) Replace method...
Replace: ^\d{3}-0
With: 35 (change as appropriate for each folder)
Use regular expressions

2) List replace method with 3 lines...
Replace: b$ with: c
Replace: a$ with: b
Replace: (\d)$ with: \1a