I'm sure with some data tweaking and date based implementation of files it could be done. I'm not saying it would be a 15 minute hack job that any old schmoe could do, but I think it could be done...for example replace the data and model for the Type XXI with Bismarck (or a Z34 or a schnellboot etc) from 1939-1944 and on 1/1/1944 replace the Type IIA data with the Bismarck data so it would be a flawless shift...now, jerry rigging the flotillas to have Type XXIs in 1939 and Type IIAs in 1944 is a different matter.