Time machine out of space / Merging time machine backup
TM will not move your old snapshots to a new drive, although I'm pretty sure that if you first used Disk Utility's Restore tab to clone your old backup drive to the new one (after reformatting it properly), it would see the old snapshots properly.

But there's a bigger problem. You are trying to use TM as an archiving tool, but that's really not what it's designed for. TM will expire old snapshots as it sees fit, so if you want reliable retention of old files it's not the right tool for the job. Unfortunately, I don't have a good suggestion for a better tool, but I would strongly recommend looking for something more appropriate.

That said, switching TM to a new drive (and removing the old one from its backup target list) would be a semi-reasonable way to retain all of the old snapshots (and having multiple redundant backups is a good idea). Doing that and switching to a new archiving tool would also be reasonable (since you probably won't be able to migrate the old TM snapshots to any net tool you select).