I renewed mine online (at 76 - no new photo requested, still using one at 70), so not a complete answer but here are the dates in my case which might give you some idea of timescales.
27th April - Reminder received from DVLA
28th April - Online process completed
10th May - New licence received
15th May - Old licence returned (as per pledge given online)
16th July - date old licence actually expired, but new one was valid from when received.
With a postal application I don't know, but I would think you have to return the old one then and there or they won't issue a new one.
P.S. I've just checked my licence - it shows start date 5th May 2018, expiry date 21st July 2021 [sic]