It should be. But the app must not be running, so close it via Settings/Applications
There are tools that can move them for you. This one is free:
https://market.android.com/details?i...nup.Inteks.org
Also, you'll need to clean up dalvik cache because system apps will get their own cache files.
And you may also have to run fix_permissions afterwards.
But: if you have system partition mounted rw and there is a crash, then there will be file system corruption. Don't panic - nothing will get lost, but the free space of the system partition might be limited until you do a e2fsck.