DS is 4 and half but he is small, 97cm tall and about 14.5kg. So not big enough to go into a Group 2/3 HBB.
The cover on my Priori is starting to look worn, mainly around the buckle not helped by DS. It's really beginning to bug me. The cost of a new cover is £60 so no not an option really.
Sensible options are:-
Try and re-stitch / patch it, use it for another year or so until DS is a bit bigger and ready for the next stage seat, probably a ISOFIX Maxi Cosi or Britax.
Replace it with a 1-2-3 Advansafix.
While he is in the harness I wouldn't have though their would be much difference in safety between the Priori and the Advansafix but is the Advansafix as good as a dedicated Group 2 seat?