Sync will not move all your data. I switched to a new profile after installing an add-on that introduced problems and uninstalling didn't correct those problems. I fell back to Sync for the new profile so I could salvage my history and bookmarks from my 2-ish year old profile, but it left out something like 9 months of history, all but two of my tags, and a good chunk of my bookmarks and search keywords (including the ones I'd set up for MXR).

I'm not sure if it was buggy, or if Sync was intentionally not tracking the stuff that didn't get copied over. (cf mconnor's blog post a month or so ago about implementing Sync quota caps).