I lay claim to around 1700 myself (incl. HD-DVDs, but that collection is small). I will rebuy things that I really like - but I wouldn't rebuy every single title for a better version (particularly since some of them I wouldn't even rebuy as they are after collecting for 9 years). A movie I really like I would definitely upgrade to an anamorphic transfer if I had the jones to watch it (maybe even if not). I double (some even triple) dipped on a number of my Argento's just BECAUSE (and I sure hope the new Tenebre and Phenomena discs will have anamorphic transfers). I think at the start of my HD-DVD collecting I did rebuy a few that I may not have considered really had HD-DVD had a much larger catalog (and interestingly the best HD-DVDs that I bought that were well worth the purchase A/V quality wise I didn't even have yet).
I think I'm babbling... But yeah - I will double dip, but I don't have to replace EVERYTHING.
I almost feel like I didn't really answer your question
As an aside, you did just get me particularly interested in checking out Gertrud soon. I have watched both Day of Wrath and Ordet and thoroughly enjoyed both but for some reason still haven't watched Gertrud strangely (it's been probably a year or two also - though actually did consider it recently).