In all the time I've played SH4 I've never been offered a new command, a tower refit, nothing! Its a real immersion killer to still be cruisng around with an old 'bath-tub' tower in the middle of 1943, when I know for a fact that boats got new tower additions/refits practically everytime they returned to base.
The devs really screwed up on this IMHO, why change the ability to buy upgrades through renown points when it worked so well in SH3? Doesn't leave much incentive to gain renown points if all you can buy is a new DG (and don't get me started on the bug that stops you changing its position, unless your willing to leave it un manned!).
If I knew enough about programming etc I'd have a go at modding a solution to this. Hopefully if patch 1.04 becomes a reallity the devs will have sorted this out! But frankly I'm not holding my breath at this point.