It sounds like your Access MDB has tables only in it. If so, you can use the Upsize Wizard to convert the data tables from ACCESS 2000 to MSDE 2000. Then, just discard the upsized ACCESS MDB, which will just be a shell of Linked table entries pointing to the MSDE 2000 tables.
Then... the customer's software program will have to be changed to reference the tables in MSDE instead of ACCESS -- but it sounds like you've planned for that step already.