It wouldn't surprise me if the Rams have verbal agreements with both AD and Kupp to restructure their deals. But those restructures come with a cost, and maybe the Rams want to make sure they need the cap space before they create it.
Typically a restructure means converting some of the current year salary to a signing bonus so most of the cap hit can be deferred. But the trade-off for cap relief this year is bigger cap hits in subsequent years.
Perhaps the Rams don't want to assume the cost of those restructures before they need the cap room. It would be a mistake IMO to clear $15M in cap space only to find that there's no one to spend it on.
How I would do it: come to terms with Clowney (or whoever) first, then do whatever restructures you need to do to fit that new deal in, and then sign your new FA.
AlbaNY_Ram