Changing RAID levels usually wipes all existing data (some RAID software will do it nicely). The simplest thing is to copy them to an external drive, _verify_ the copy, and then change your RAID. Copy the saved data back.
Two suggestions. First of all, the Time Machine software will probably let you browse your backups and delete older data, perhaps even set a policy about the max number of copies / max amount of data usage allowed.
Second, while I know nothing about that NAS, or any NAS, RAID1 can often be "broken," removing one drive from the RAID set and leaving the other as a standalone drive. If the management software will let you do this, you just reformat the removed drive and use it as a second drive, or even span the first onto it. I would recommend using it as a second drive rather than spanning. One for your data, one for Time Machine, and the one can't eat space that you meant for the other.
Hoping that this helps