Does the damage prevent your machine from working properly? If not, I'd suggest not replacing the case -- the aluminum towers (especially the G5s) are constructed very specifically, and you run a very likely risk of rendering you machine completely inoperable by performing this transfer. A working machine with cosmetic damage is much more preferable to a pristine-looking machine that won't boot.
On the other hand, if the damage has a serious impact on the operability, you've got nothing to lose.