It was python from the start ( https://news.ycombinator.com/item?id=8892 ), but maybe the 2to3 migration, or just a general rewrite you're thinking of?

There was a "major" change in the agent roughly a decade ago. Dropbox went from being a simple folder sync tool to a much more bloated agent that never synced cleanly. I don't know if they just moved away from some modules that were written in C or something, but I gave up on it soon after.