For safety, you should have some periodic snapshots. And preferably some replication strategy to another NAS.
And periodic SMART tests. Etc.
So you have a mirrored pool, and enough ports in your new NAS to plug all drives at the same time.
Easiest:
Destroy your new (empty!) pool.
Go to Storage > Pool > Status.
Select the first drive in your old pool and “Replace” it with a new drive.
Repeat with the second drive.
done