I've said it numerous times and received Flak for it, but my opinion remains unchanged:
MK II fighters should receive a different model, MK I fighters can stay as they are now.
They don't even need to be drastically changed, add an additional fin, wing or engine or so. Their general texture can be retained too. As an example, this is the upgraded Crusader of the pretty old Evolutions mod: http://the-starport.net/uploads/extgalle..._8ddda.jpg
(Disclaimer: Not made by me)
I always saw the MK II ships reusing MK I models as some kind of very cheap makeshift solution until proper models get made. But they stayed ever since I played Disco for the first time.
Them having the same model just makes no sense RP-wise either. You cannot boost hull strength, power output, let alone agility by exclusively upgrading interior components. Yes, there are military aircraft that get updated and look pretty similiar to the base version, but 1. they do not look exactly the same and 2. they do not receive upgrades that improve their durability and firepower fivefold like on Disco.