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I actually was looking at this recently. I don't have my notes available, but I'll try and cover what I remember.
As Mamba indicates, a large portion is dependent on the modules. A lot of them will have at least a common ancestor, so it is usually not hard to find a workable replacement. Even for things unique to ?????, some creativity with SQL can extract content in a form usable in another module.
The user conversion is a potential issue depending on the size (and temperament) of your user base. The password hashing has several salts, and is quite different from anything in any Xoops, current or future. The cleanest way to do it is basically mass invalidation, requiring every active user to go through the password reset mechanism.
The system I was examining was in the 1.2.x line, the 1.3+ variants introduced additional differences, but are still similar.
I would encourage you to keep us posted on your efforts. There is a certain level of animosity that you might run into occasionally regarding mentioning our competing "cousin." Personally, I have held "a best tool for the job at the time" attitude over the years, and so I've used a lot of things. But now, if XOOPS isn't the best tool for the job, that means I have work to do
If there are any major obstacles to moving back to XOOPS, I'd like to know what they are, and if practical, make them less of an obstacle.