We have 3 different numbering systems over the course of our organization’s history. One system was similar to the one you described. At one point, the old numbers were given modern numbers. We use a numbering system with the year and catalog number and separate accession number (so the numbers look like this: 2013.1 / 211).
To renumber the old collection, we gave it each lot a (since we knew they came in as one donation) a modern accession number. The catalog number given was from the year the pieces were originally donated (so things from 1935 were retroactively given a 1935.xx number). All the info associated with the old numbers was transferred and a note made on the old and new paper record of the change in number. You can also track this in PastPerfect under the “Old Number” field.
Alternatively, you don’t have to renumber at all. If the reason for the renumber is to have separate catalog records, simply add letters to the end of the number to separate the objects. For example, in the case of the 6 coin purses you would have 71.31.2 a, 71.31.2 b etc. Renumbering is a long and time consuming process and you have to be sure the reasons for doing so will outweigh the resources used and possibility of confusion in the future.
Hope that helps.