If you need the Autonumber to have specific values (a business meaning), then in my opinion, you use an Autonumber for something it isn't intended for. It is only meant to provide a unique number per each record, nothing more. When cancelling a record while typing it, you'll get gaps in the numbers (missing numbers)... how will that go when you're missing Invoice numbers?
Anyway - here's one way of altering an autonumber (think this method requires Jet 4.0/Access 2000+). Just run it in the immediate pane (ctrl+g) using your table and column names.
currentproject.connection.execute "alter mytable alter column InvoiceNo int identity (5000,1)"