If you plan on removing all of your legacy HDDs from the DNS-321 when you receive your new 1.5 TB HDD, installing the older drives will not cause any issues. If you plan on keeping one of the older HDDs in the DNS-321 along with your 1TB HDD, and format the HDDs in Standard Configuration, you will also be okay.
You will encounter issues if you configure an older (lower capacity) HDD and your new 1 TB HDD as RAID 1. The RAID 1 array will assume the size of the smaller of the two HDDs.