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