It is definitely not a "Fachwechsel". As far as I know, it may be even sufficient to just tell them (the Prüfungsamt) that your exams in Computer Science should be handled instead of the version of 2009 by the examination rules of 2018. But in any case, you should first discuss the change with your mentor and maybe also with the examination committee, since you cannot change back to the old system in case you would regret the change.
I am not absolutely sure whether the same holds for Applied Computer Science, but I haven't heard anything different so far.
