Hi mate, I think you can upgrade Thunderbird with no worries as it keeps your messages untouched. Although I haven't tested that in Linux (where I'm writing now from), I have done it under Windows (at my office where I work in) and I didn't have any problem at all. Since Thunderbird is multi-platform, I suppose you shouldn't have any problem at all. Just installed the program as you'd usually do and it should be ready.
I hope my help to be useful for you...
Good luck!
