Since few folks here are actually speaking with any facts, I will since I have some experience doing this with other models. The Modern Sporter and SLR, like any other platform not restricted or prohibited by name/variant, is either restricted or non-restricted based on barrel length.
When changing the status of a firearm you have 30 days to report the change. Until your firearm has either been successfully removed from the registry in the case of conversion to non-restricted or properly registered and registration issued in the case of conversion to restricted, you must apply the more stringent regulations and rules of restricted status to your transport and storage procedures. It is important to note that you are not permitted to use the firearm until the CFC has been made aware of the status change.
There are many folks who suggest that it is fine to change quick swap barrels at the range but I sure wouldn't want to be in their position should they get caught. The law is fairly clear in that you must notify the CFC of any changes in status.
As far as having a restricted length upper that is compatible with a non-restricted lower while not being in possession of a registered lower to accommodate said restricted length upper, I would imagine it would show intent, and as such I would not recommend being in possession of a restricted length upper unless you have a compatible and properly registered as restricted lower. If that means you disassemble it and put it away in pieces when you have your singular lower un-registered, so be it. Now if you had your singular lower registered as restricted you could in theory have both a restricted and an unrestricted length upper, however so long as the firearm is registered in the CFC database as a restricted firearm and you haven't gone through the process of getting it deregistered it must be treated in accordance to the law as a restricted firearm.
I am not a lawyer but I do have a fairly strong understanding of the regulations and processes. I would suggest seeking legal advice before going any further to be sure you have a proper understanding for yourself of our laws and regulations.