Agreed with chrfr about making a backup before you try this conversion.
The BEST kind of backup would be a bootable clone created with either CarbonCopyCloner or SuperDuper.
That way, if anything ever went wrong, you could just do this:
1. Boot from the cloned backup
2. ERASE the internal drive
3. RE-CLONE the cloned backup BACK TO the original drive.
4. Voila! Right back "where you started from..."