My experience is that if I do a LoTW pull with a date of, lets say, two days ago I get a QSL from a QSO I had maybe two months ago if this QSL is confirmed by LoTW in the last two days. So if you would do a LoTW pull every week with a date selected one week prior, I think it will work. If one would sign-up to LoTW and upload their entire logbook, LoTW will try to match every QSO (as far as I know) with QSOs already known by LoTW.
In other words, you don't have to update from the very beginning. Again, this is just based on my experience with LoTW. I am by all means no expert. 🙂
Would be nice to be able to do this LoTW pull via cron every night or so 😉