I just bought an HD500X today. I bought it as a backup for my HD500. Basically, what I wanted was another HD500 so I didn't have to futz with resetting all my sounds or translating the files. The on-line retailers have basically dropped the 500's for the 500X's, except I found GC's online catalog still has them at $399, so I held off until a holiday (Labor Day) when they have their sales. I checked the local GC and they had it, but when I walked in, they were marked "clearance", so their 15% off wouldn't apply. With 15% off an HD500X, it was only 25 bucks more than the HD500, so I decided to go with the 500X.
I hooked up my 500 to the HD500 Edit software, saved all my patches, all my set lists, and a bundle. I then hooked up and set up the software for the HD500X, updated, yada yada.... Anyway, when I got to the point of hooking up to HD500X Edit, I had copied my set lists into 500X Edit's directory for it and renamed the set list's extensions from .h5s to .5xs, and opened them with HD 500X Edit, and they all loaded without a hitch. I was very relieved at how easy it was. They all sound exactly as the do in the 500. When you start to see 500X patches that use more DSP start showing up in the custom tones, I kinda doubt you'll be able to bring it back to the 500 and work, though..
The new one will be my main one with the 500 as the backup. I've been taking my X3L as my backup, but every change I make on the 500, makes the 2 drift further apart for backup purposes. I did have my XTL die at a gig with no backup, which is when I got the X3L... The night of the gig though, I was dead in the water, had to run home and grab my regular guitar amp, but it was not easy to switch in & out of acceptable clean/dirty sounds... We had to start about 45 minutes late -- not a pretty sight.
I am an engineer, and do believe in contingency plans, because sh1t does happen -- been there, done that... As far as I know, the patches will work the same by just renaming the extensions for 500 to 500X patches anyway... .
My 2 cents,