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Based on my understanding ,your DFS-N server and DC are not on the same server, there won't be any differences for changing which firstly.
But before any change ,i would recommend you make sure that everything works well for the current situation and do a backup for both the servers.
For the DC migration , promote the new one firstly, after the replication completment, run DCDIAG and Repamin on the new DC .If all is good, transfer the FSMO. For your reference:
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/upgrade-domain-controllers
Add Namespace Servers to a Domain-based DFS Namespace