We had the same issue earlier this year when we decided to convert our Excel inventory to DB software. We concluded it would be a huge amount of work to properly renumber a sequential system to AccessionYear.Accession#.Object#. Also, we didn’t any accession paperwork for many objects. Instead we used for everything the new format with 1983 as the accession year (when we were founded), made the second number 01 and let the object number be the third position. So we kept the old numbering but within the new format which we will use thru the end of 2013. Starting 1/1/2014 we will use the actual accession year and accession number within year.
For objects with the same number but letters we dropped A’s and replaced B with .1, C with .2, etc. We reserve letters for single objects with multiple pieces.