Can I replicate to a pre-existing database?
Yes. After dropping the subscription, you will be able to simply configure replication from the new publisher to the existing subscriber. There is no need to drop the subscriber DB. Please bare in mind however that the subscriber will need to be re-initialized from the new publication. (Assuming transactional or merge publication) this can be achieved either through the snapshot agent or a backup/restore. 

Also bare in mind that assuming your new publisher will also be your new distributor, you will need to configure this as well.