MegaSquirt has a fairly good Support Forum that you should find the answer to the upgrade question on. Typically their upgrades are firmware based upgrades that can be installed by the user. If you go to the support drop down on their ampefi.com website or their diyautomotive.com website and drop down to contact us there is a phone number you can call to talk to a real person.
Whie the MS1 and MS2 units are great starting points you might want to investigate their MS3Pro-Ultimate units. The difference is sort of like an electric gatling gun vs a single action army colt pistol. The MS3Pro-Ultimate offers significant capabilities beyond the MS1 and MS2 system capabilities. From a feature standpoint it is similar to systems like Haltech or Motec costing 2x to as much as 5x the price.
I understand that, I have an ms1 unit right now, when I played with the og 2.3 turbo motors I ran megasquirt and had it upgraded to an ms2 unit by a member on the forum that specialized in megasquirt, I thought maybe someone on here was the same way which is what I was asking, for what I'm doing the ms2 will work great, it supports edis8 and can even be made to work a 2 step and boost controller, and the ms2 can be upgraded even further if the need arises.