I think that setting properly your POP account will be enough.
Your contacts are easily exportable. Just save them as a VCARD
addressbook (or separate VCARD files) and import them in the new app.
I can't encourage you to make the change. Time ago I try Claws Mail but
I didn't like and never try it again.
Exactly, why do you want to change? What is wrong or what is the lack
you find in Thunderbird?
You can also try: Seamonkey (Mozilla), Evolution (with calendar and
other things), Kmail (a little tricky but with good features).
Regards,
Lailah