Rename Multiple files in unix

You were right to consider rename first. The syntax is a little strange if you're not used to regexes but it's by far the quickest/shortest route once you know what you're doing:

rename 's/\d{4}/2503/' file*

That simply matches the first 4 numbers and swaps them for the ones you specified.

And a test harness (-vn means be verbose but don't do anything) using your filenames:

$ rename 's/\d{4}/2503/' file* -vn
file0901201437404.p renamed as file2503201437404.p
file0901201438761.p renamed as file2503201438761.p
file1003201410069.p renamed as file2503201410069.p
file2602201409853.p renamed as file2503201409853.p
file2602201410180.p renamed as file2503201410180.p

a) find + prename(Perl rename) solution:

find . -type f -name "*-XYZ12-XYZ12-*.gz" -exec prename 's/(-XYZ12)(\1)+/$1/g' {} \;

b) Additional bash + find + sed approach if prename is not supported:

for f in $(find . -type f -name "*-XYZ12-XYZ12-*.gz"); do 
    p="${f%/*}"      # full path without basename (parent folders)   
    fn="${f##*/}"    # current filename (basename)
    new_fn=$(sed 's/\(-XYZ12\)\+/-XVZ12/' <<<"$fn")  # new file name
    mv "$f" "$p/$new_fn"
done

c) Also, you are able to avoid using sed in the above bash approach by using just bash variable substitution:

shopt -s extglob
for f in $(find . -type f -name "*-XYZ12-XYZ12-*.gz"); do 
    p="${f%/*}"      # full path without basename (parent folders)   
    fn="${f##*/}"    # current filename (basename)
    new_fn="${fn/+(-XYZ12)/-XVZ12}"  # new file name
    mv "$f" "$p/$new_fn"
done