On my Win7 system, ACR 9.2 is forcing a re-cache of all Bridge thumbnails.
The Bridge thumbnail database has been compatible for at least versions 8.6 through 9.0. I could switch between those versions with no problem. Now, when I launch Bridge under ACR 9.2, every old folder I point at gets re-cached. Regardless of file types. Jpegs, tifs, cr2s, nefs, dngs, all get re-cached.
If I switch back to ACR 8.6, 8.7, 8.8 or 9.0 they each will re-cache anything done by 9.2. Let one of those older versions cache a folder of images, then switch back to ACR 9.2 and they get re-cached again.
There is a long history of problems with Bridge re-caching thumbs. We discovered long ago that ACR is the root of those problems. To decide if a particular image needs to be cached or re-cached, Bridge must look at the file/folder name plus date. If the cache does not have an entry matching that "tag", a new cache entry needs be generated. It seems apparent that Bridge uses ACR to get one side of the equation, and something else to get the other side of the equation (probably the OS). Those two sources sometimes don't match when, in fact, they should.
I suspect the problem involves date/time. I'd bet the date/time values involve some high bit floating point math. If not done properly, quantization or rounding errors can cause two identical floating point date/time values to compare unequal.
I realize I'm spitting into the wind here, but hope there is a remote chance this theory can reach an Adobe tech who is willing and able to investigate. Suffering a re-cache on a library of thousands of images is a PITA, and totally unnecessary.