The two big things that were upgraded on ships during the period were AA (and for the big ships, this also often involved the introduction of better dual-purpose secondary weapons - the US 5"/38 for example) and radar, and literally every RN or USN ship from CA up built before they were standard received substantial upgrades of both during the war. There were a few other odds and sods (the UK added some cemented belt armour to some of its CAs (I think - might have been CLs), for example, and the Independence class CVLs were built from CLs under construction), but aside from Japan, most ships didn't have substantial changes and the game can get away without having conversions in (although I'd love it if they were

). Japan's the big exception - they changed CLs to CAs, AVs to CVs and did all sorts of things to ships under construction, but given the faster ship-building times in HoI4, it won't be as necessary in-game as it was IRL even if Japan loses Midway again.