I’m not sure if this will help your situation, but my museum has merged collections with two other larger collections, both of which had some cataloged and some uncataloged items. The cataloged items used the same numbering system that we were already using, creating duplicate numbers. Our solution in this case was to add a letter indicating the old collection and the year the gift/merger occurred in front of the old number. It makes a longer number (W.1922.214.171.124 for the Wentworth Collection, and H.2008.55.7.14 for the Historical Society) but you don’t have to totally renumber hundreds of items and their records. And we can tell which collection the item is from just be looking at the number, which is helpful when a donor wants to know if a specific item is the one given by his/her family to one of the collections.