We still use the old method to auto increment a number by having a rule in the BO like:
If Invoice IS NEW Then
Invoice.InvoiceNumber=MAX Invoice.InvoiceNumber +1
Please note this can (in rare circumstances) be a problem if 2 users create a record at exactly the same time. It does happen - it happened to us.
Support has confirmed that the new auto-incremented fields were created specifically to avoid this problem - so you should use them instead.
FYI - we have found taking a field from the old method to the new method does not affect the existing data and the incrementing continues from the existing number, so it is very easy to implement.