Depending on the edits, they may get broken as there were some major changes with the release of RC6. What you can do is use the automatic update packages for RC4 to RC5, then the RC5 to RC7, and finally RC7 to Gold.
Note that there is a special upgrade package which skips RC6, as there were a few issues with going from RC5 to RC6. There was also one to skip RC8 and go straight to Gold.
So here's the list of downloads:
https://sourceforge.net/project/showfil ... _id=194469
Grab the following:
- phpBB-3.0.RC4_to_3.0.RC5
phpBB-3.0.RC5_to_3.0.RC7
phpBB-3.0.RC7_to_3.0.0
Hopefully that will keep your edits intact. The updater will ask you if you want to use new files or to merge, choose the merge option.