Yes, you can fit a new ''goalpost' rubber (the sides and top in one piece) and the lower moulding without removing the screen or retainers. I did it last year and it sits perfectly.
Upper moulding (goalpost) 90460335
Lower moulding (base of screen) 9175100
Both are becoming a bit scarce but were still available last summer, because I bought a set. Autovaux are cheaper than VX for genuine VX items and they will deliver it. The goalpost mouldings are in a large box for protection and won't go in your average saloon.
I have found the best way to remove them without damaging or loosening the retainer is to use a right angled pick tool, and once one end of the tongue on the moulding is started, slide the pick around the moulding between the bottom of the tongue and the retainer to release the tongue. This minimises the pressure on the retainer, which may be damaged if you just try to yank it out.
Prior to reassembly, carefully clean out the groove in the retainer of grit etc. I put a thin smear of red rubber grease on the tongue which helps it to slide into the groove in the retainer. Without a bit of lube, it is easy to damage the tongue which itself is thin hard plastic. Endsuring it's correctly positioned relative to the top corners, just feed it in with your thumb all the way round and bobs yer uncle!
A helper to hold the goalpost moulding makes it a bit easier.