if you have a string like "6FA34323-C71F-4FD4-996F-002CE77E6B77"
search w/ regexp using ("[A-Z0-9-]+") replace with \L$1
\U will uppercase
Vim version:
%s/\([A-Z0-9]\{8}\)-*\([A-Z0-9]\{4}\)-*\([A-Z0-9]\{4}\)-*\([A-Z0-9]\{4}\)-*\([A-Z0-9]\{12}\)/\L\1\2\3\4\5/gc