If you're still managing the AVCHD drive in AVCHDManager, you would just click "AVCHDMe-undo". All it does is rename all the files back to their full names i.e .mts->.m2ts, .cpi->.clpi etc. If you're not using AVCHDManager, there's a standalone version of AVCHDMe_undo
here. Just place in the root of your movie folder and run. Mediafire is down for maintenance, check back later when they've finished pissing about.