For what it is worth, I asked my travel agent this type of question some time ago when I booked a solo rate, knowing about the potential for somebody joining later.
Basically the answer was yes, it should be possible. Only the difference between the rate paid and the double occupancy rate (if any) for two would be due.
Yet, the need for a change did not materialise, thus it is not practical experience but just a theoretical answer given by one TA a few years ago.
Now your upgrade example is obviously more complicated.