Access Import Issue

I imported workorders into my main table and it caused a problem with the workorder numbers which is an autonumber. It went from 7000 to an 9 digit #. So I was told that I should not be using autonumbers for anything that is meaningful to the users, change my workorder field number to a long integer then populate the WO field in new records using code which determines the highest value, and adds one to that value. Any idea what that code is? My table is called "main Table". My field for the workorders would be "WorkOrderNumber".

The formula I was given was: = Nz(DMax("[IntPurchaseOrderNumber]", "tblPurchaseOrders") + 1,1) but obviously this doesn't go with what I have as my fields or tables and even when I change it to my field and table name, I get a name error. What am I doing wrong?

This isn't a good thing, as my boss started this database back in 2004 and it's been running solid since then without a hitch and they don't teach you in school, that the autonumber can't be a "public field".

Learning as I go.