This happens to me when I'm importing photographs, I end up with files called .JPG or .jpeg and I prefer .jpg.
Say you have some files named a100 to a200 that you want to rename to b200 to b100 (it really does happen).
y=200 for x in `seq 100 200` do mv a$x b$((y--)) done
seq is a fairly useful tool, it can also take a reversed list.
for x in `seq 200 -1 100` do printf "$x " done