hey alree, have a look at "Restorator 2007", it can create a "Patch_TTSRes16.exe" with only your modified graphics inside. then the user can slap the patch into their sonar directory and if they are using "TTSRes17" they can rename it to "TTSRes16", patch it, then rename it back to "TTSRes17". as you mentioned the numbers stay the same so shouldn't be a problem. and the patch itself would only be about 120kb +/- (as it only contains the bitmaps)
EDIT: when the patch is used, it also creates a backup of the original file prior to patching.