I get where you're coming from as far as wanting to use some unit of measure for replacements.
If size is our justification, then we should use ship length. I don't like using the cost because pretty much any ship you're going to want to upgrade to is going to cost more. A TIE is 7m and costs 48MC. A TIE Interceptor is 7m and costs 54MC. Why shouldn't you be able to do a 1 for 1 replacement here? With length, you can, with MC, you can't. If we want to go for some realism there, then I think we should stick to length.
That said, at some point we shoot ourselves in the foot for realism vs simplicity. I absolutely agree that it makes sense and is more realistic to have some sort of comparative measure for replacements. But, at the same time, that adds a rather significant level of complexity, too. Is that complexity worth it? I lean towards no. If you're really concerned about it, then I would propose to classify fighters into 3 groups. Light, Standard, Heavy, then use the table below. A TIE would be light, an x-wing standard, and a b-wing heavy, for example. Say, <=10m is light, >10 to <=16 is standard, and >16 is heavy.
| | Light | Standard | Heavy |
| Light | 1:1 | 2:1 | 3:1 |
| Medium | 1:2 | 1:1 | 1:2 |
| Heavy | 1:3 | 1:2 | 1:1 |
So, within class it's a 1:1 for squadrons. Moving up a class means you can fit an additional squadron. Moving down a class means you can fit half the squadrons. If you're worried about swarm tactics/too many fighters then make it a one way conversion by saying you can never have more total squadrons than you start with, like AUX are now (and we probably should have that, even though it reduces options).
Using the VSD as an example: The VSD gets 2 TIE squadrons (a light squadron), so a VSD could carry 2 TIE, 2 TIE/I, 2 IRD/A, or 2 A-Wing squadrons. OR, 1 TIE/A, 1 TIE B, 1 Z-95, 1 CloakShape, 1 R-41, 1 Hornet, 1 X-Wing or 1 Y-wing squadron. OR 1/2 B-Wing squadron.
The DRED would look the same (starting with 1 standard sized squadron), unless you want the rule of no more than the starting squadron count. In which case, it can have 1 of any light or medium squadron and 1/2 of any heavy.
The Venator starts with 5 Standards, so you could do something like this as an upgrade: 2 IRD/A Squadrons (1:2), 2 CloakShape (1:1 exchange), and 1 B-Wing (2:1). Still 5 squadrons, still basically the same amount of space accounted for.
This approach gives us some level of realism while significantly reducing the number of calculations that need to go into it. I would say with this approach that you keep the starting squadrons as is and we pay the difference for replacements. It also means that ships don't need to be re-priced again.
TL;DR: 3 Sizes of Fighters. 1:1 replacement within sizes, 1:2 / 2:1 between adjacent sizes, and 1:3 / 3:1 between the others. Limit of same number of squadrons as started with. Upgrade cost is replacement fighter cost - base squadron value at build time. Sizes are <=10m is light, >10 to <=16 is standard, and >16 is heavy.
P.S./Side Note: I'll start another thread to discuss mothballing.
