Thanks for all youre answers.
I did some more looking around and i found this page
http://alexbret.perso.cegetel.net/index4875.php
pretty much told me which upgrades were available for which boats at what time etc...
My usual campaign (GWX 2.1) starts in 1939 in type IIA which i keep until i can afford a type VII B or C, then later on in 41/42 i buy a type IX. i like playing that way as in the early missions you do lots of patrols so you end up with a very specialist crew core (one speciality per patrol) to which you add the extra crews when you move from class to class so in my longest campaign i had an elite core which had been with me since the beginning (apart from most of a deck crew which got killed back in 1940) which have i have given most of the promotions and medals.