reset autonumber for new records

The short answer is: you can't.

Access intends for the autonumber field to be a number useful only internally to the database engine as a unique record identifier, but not have any other meaning to the programmer. It is bad Access programming practice to try to use an autonumber field for any other purpose (like concatenation with another field) unless you accept the fact that you have little to no control over the value assigned to the autonumber.

In your situation, as long as you have a field with an autonumber value of 453,xxx,xxx the next autonumber will always be 453,xxx,xxx +1. The only way to reset the values would be to remove the records with the offending autonumber values and then compact/repair the database. In theory, after the compact/repair, the next autonumber value will be the highest existing autonumber value +1.

It would be better for your purposes not to use an autonumber at all, but instead create a small procedure which fires upon creation of a new record and assigns a unique record id. That way you can be assured of having complete control over the numbering system...