Before Samva touches your DNS, you see exactly what it plans to do. The domain page lists each record as one of three things:

Will add: the record is missing and will be created.
Already there: it exists and stays put.
In the way: another record holds that name. Samva never replaces it, so fix it at your provider and check again.

Select Add N records to approve. If your DNS changed since the plan was built, nothing is written and you check again. A partly applied plan is safe to retry.

You can also import a connected provider's zones as domains without changing any DNS, then set each one up when you're ready.