One option is to create a table with EXACTLY the same fields as your existing. Let's call them DUMMY and KEEP for the moment.
OK, in DUMMY, create 299 records. (Just put junk into the first data entry field)
Build a query based on KEEP that has every field except the Autonumber field.
Turn it into an Append query, and append the records to DUMMY.
Now go to DUMMY and delete the first 299 records.
Delete KEEP and rename DUMMY. Note: You'll need to delete any relationships to KEEP before you delete it, then re-establish those relationships afterwards.
There are other, code-based solutions around, but this quick and dirty method does the job.
Denis