There are 2 issues: 1. Planning permission - which tends to be concerned with the street view of the house - and was probably not required. 2. Building Control checks that the extension complies with Building Regulations . If there are external doors separating the "conservatory" from the main house then it is like an "outbuilding" and does notneed building control consent either.
So it sounds all OK - you can just call the local authority Planning Department to check.
Replacing the roof - different question. Glass might be heavier/ thicker. You will need to get in an expert to look at it. It may be simple or may need some of the structure replaced/ strengthened.