As a matter of planning strategy, decide if this move is a short term fix, or part of a long term transfer of role. Another consideration is do you want all the roles on the same Domain Controller. The answer is probably not, for example, best practice suggests that the Infrastructure master should not be on a Global Catalog.
Your planning should also take into account the fact that each domain has its own RID, PDC and Infrastructure Master, while there is only one Schema and one Domain Naming Master for the entire Active Directory Forest.
So far our Serveris running ok with the current configuration, and it is simplifying the maintenance in case of Server A rebooted.
but what I'm afraid of is that the First DNS server demotion might fail in the middle of something then I'm in a very2 deep trouble :-( as it may render the whole company useless.