Try and keep an eye on any potential platform alterations ahead of time, as Kite says, there are some good long walks with the footbridges being at the west end of the station. A 3B to a 4 or a 7 to a 1B are not super fast.
There is no reason that someone able bodied couldn't move fairly swiftly from one extreme to the other in 7 minutes and if you only need to do a 3B to 3C then that should be easy. Bear in mind that the Norwich train divides at Nottingham, so this can take a bit of time, but there is padding in the timetable to allow this so if it is late arriving it won't necessarily be late leaving. It is due into NOT at 1532, but these services are not know for their prompt running and as your service terminates, if the Norwich train is going to arrive after yours due to late running then one of the 2 is almost certainly going to be booked into a different platform at late notice.
If you fancied a change of train, scratching a few more local shacks off, and your ticket permits it (which i assume it does) Then you could always get the 1623 out of Nottingham which is Skegness bound that calls at Grantham about 15 minutes before the next Norwich service.