Hello Mario,
The machines already activated with the 2016 KMS will attempt to activate with that machine. A KMS client will try to go back to the KMS host it last activated with first5.
If that fails it will query DNS for another KMS host to Query..
Reasons why the existing KMS clients may fail to reach the KMS2016 host include but are not limited to:
Network issues including DNS issues.
Lack of count of the KMS 2016 Host.
KMS Host is unavailable, rebooting for updates for example or down for Maintenace .
The bigger question is how many KMS clients do you about? Roughly.
100 or more there shouldn't be any concerns.
You can use SLMGR to specify the KMS host to query thus overriding the DNS query for any KMS host.
From an elevated cmd prompt:
you can run: ( replace "Computername" with teh name of the KMS host you want the KMS client machine to activate with.
slmgr /skms "computername"
See slmgr /? for all the commands.
The above command to specify the KMS host name can be reverted if you need to go back to having the clients query the DNS host for KMS hosts