They will either have to renumber the HHP-8s into a different block or numbers the ACS-64s leaving a gap in the middle for the HHP-8s. I think they will renumber the HHP-8s since having two classes o0f engines with overlapping number groups will be darned confusing. Or of course they could retire the HHP-8s before the problem arises.
With the HHP-8 numbers starting at 650, Amtrak could retire the remaining HHP-8s sequentially as the same numbered ACS-64 is delivered. Amtrak will have 50 ACS-64s in house before the numbers conflict, so keep some AEM-7ACs around as the HHP-8s are retired until the production run of 70 units is completed. Probably simpler, though, to renumber the HHP-8s to 750 and up, if those numbers are open.