The latest development roadmap from Asobo shows an SDK update in late September. So, it's unlikely the SDK will be released in a completed form before then. So, only approved 3rd party developers will have access to the SDK until at least that time. So, we will all be in the dark until sometime after September, and hopefully it will be free or reasonably priced. I have read the SDK is a little sparse as of now, but they are working on it.
The sim's release will be August 18th. I read it will be upgradable if you get the cheapest version. The more expensive versions contain more default aircraft and detailed airports, so essentially you would be buying DLC. But the cheapest version will be complete. Associated programs like Little Navmap should fundamentally work through Simconnect. And I read the Dowsons are working on an FSUIPC version.
We'll need to be a bit patient for the tools we need for development, but essentially, the same elements are there. Objects, autogen, imagery, flattens, elevation BGLs, aircraft, living world. There was mention in earlier posts about migrating FSX items into the sim. I think this just relates to gauges translated through web assembly (you'd need to look that term up). As far as I can determine, that is the only migration possible, and you would probably need the C++ source code to migrate it. I think all gauges will need XML, or a combo of Simconnect or FSUIPC to function.
I do not think we can simply use a BGL from FSX and expect it to work. So there is lots of room for development. 40,000 airports to rework, tons of the earth to detail with imagery and autogen. I have no idea if objects have conditional display available (I doubt it as the ortho-imagery just has Bing and snowy winter as seasonal display). Aircraft developers will have a new clean slate for development of nearly any aircraft. All the addons for FSX and P3D will not work, so if you are concerned about your past 'investment', it's not good news.