cant change just the 5stud bearing carrier assemblies, it throws out your geometry. the ball joint is further forward on the 5 stud setup so it messes up the castor. you can press a 5 stud hub into a 4 stud bearing carrier, but this means you have to re-drill 4 stud discs or buy early passat vr6 discs which are VW only:
357 615 301 A - 280 brake disc, 5x100 - 38.83+VAT
lot of work when you have everything you need just sat there ready to fit! can swap just the rear stub axles over to get rear discs, but you wont get the roll bar then. dont count on the 5 stud beam being wider though usually its only early 4 stud ones that are. either way, make sure you fit some kind of bias valve, without it the rears will lock up too early which is bad. if you are lucky the mk3 has a bias valve, it probably will being an early one. if not you'll have to strip the bits off a mk2 gti and change your brake hoses to get it plumbed in.
you can also fit all the mk3 alt/pas/belt setup from it, way better than mk2 stuff.