Well, there will be new systems as well, but I meant a new 'tier' of aircraft will also be unlocked for you to choose or upgrade, if you so wish. 
I'm not going to use a money system, as that's far more book-keeping than I can be bothered to keep track of, and I don't want to have to assign 'values' to aircraft or to peoples' achievements and give you banana money based on them. So yeah, it'll pretty much be a case of just swapping things out. I'll have a think about it, and maybe I'll introduce some form of 'slots' system for upgrades, representing the systems you're able to upgrade, and then introduce systems you can swap in and out that take up different numbers of slots, and aircraft having certain numbers of slots, and based on a tiered 'tech level' for various aircraft and their variants, so only some planes can take some systems, etc. 
I could do the same for weapons as well, meaning to get the really good stuff upgrades will be needed, but not essential, allowing you to keep your existing planes if you want. 

I'm just spitballing at the moment, but that seems like a good idea to me at the moment, considering we aren't using any kind of stats right now.