I work on some RH projects with two other tech writers. My PC's hard drive failed. I suspect I may have had some RH topics checked out at that time in RH 10.
My replacement PC came with RH 2015, but did not have RH 10. I copied my local RH folder copies from a backup of the old PC to the new PC. I upgraded some of our projects from RH 10 to RH 2015 successfully. On other projects, when I tried to upgrade from RH 10 to RH 2015, I got the message, "Cannot checkout all the files that needed to upgrade this project." (and, yes, that is the exact wording of the message).
Our project admin determined that I had the files checked out (probably on the PC that had failed) that were preventing the project from being upgraded. He removed the locks from those files in source control. I tried to upgrade the project again, and I got the same message. I took a new copy from source control, tried again, same message. I changed the read-only property on my local PC's copy of the files to not be read only for the files source control previously detected as being checked out. Same message.
One of the other tech writers, who now has both RH 10 and RH 2015 on her PC, tried opening the project in RH 10. Attempting to open the project from her RH 10 screen, she received the message, "This project is from a newer version of the program. This project is no longer compatible with this version. Please make sure you have the newest version installed." She then tried to open the project using RH 2015 and received the same message I did.
At this point our backup copies do not seem to be helpful because the .xpj file--at least in source control--appears to now be identified as an RH 2015 file, so we can't go backward to the RH 10 version. At present, our attempts to complete the conversion to RH 2015 have been unsuccessful.