You can either close the YouTube channel (go into YouTube settings / Overview / Advanced / Close account) which is auto-created on the Google account you created to link your legacy YouTube to or you can try to go in to the creation process via the log in to the unlinked legacy YouTube where you are required TO link. Not having done this since we got the *@£%*! Google accounts from hell I cannot say if this will override the creation of a superfluous YouTube but here is hoping. Unless YOU have discovered that when you try this in the latter fashion you DO end up with another pointless and unwanted YouTube?
Let me know- it is a serious issue and uncomfortable oversight on part of Google who seem to not be interested in supporting us legacy YouTube owners any longer.
After ALL that- WHY do you want to unlink it from its Google account at all? It is only necessary where you wish to use a specific Google account you already have, with a different set of services on it you wish to "merge" under one Google account. Because you can just rename its own Google account and that is SO much easier (that is- unless you are using GMail in which case you are going to find that though you unlinked it you cannot link it to anything BUT that GMail Google anyway...). So- relink your YouTube to its Google account, go into Settings at www.google.com and give the Google account a new email address. Done.