Topic: In- decrease the ascii value

Find the ascii value of an char (letter and digit)
and decrease this value (or increase)
and give back the new value:

I have   -  I want +1    -   +3      -  -4
A          -  B                 -    D      -   X
f           -  g                 -    i        -   b
z          -  a                 -    c        -   v
1          -  2                 -    4       -    7

Also find %N  and in -decrease by n  would be needed?

Useful for some funny renaming
and e.g. for to increase numbers in file names.

f.ex. Falco.mp3      => +1    =>  Gbndp.oq4
f.ex. News-003.doc => +1    =>  News-004.doc

I can do this manually but for different values (plus 3, minus 5) i have to modify the expression every time.

Alternatively we must export the names to an editor, use there an script, and re-import the modified names.