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

---