I tried the free version and I think it found what I need. Is there a limit on how many files can be recovered with the free version? And if I need to upgrade to pro, can I just pay and put in the license code into the free version?
No, piriform does not hobble their free versions. The is no limit to times or number of file. A few extra features are available with Pro (such as the ability to first save a copy of your source drive as a virtual-HardDrive file (vhd). Mostly the pro version gives you priority develop-direct support instead of needing the forum.
If you do buy the Pro version, yes you can just put the registration in the free as you asked.
Thanks for the info. Unfortunately after doing a deep scan and selecting thousands of files that were rated as "Excellent", only 100 were recovered.
That has less to do with free vs. Pro than deep scan showing files that weren't currently overwritten, but had perhaps previously been overwritten