Creo que es hora de que cambie mis planes.

English Translation

I think it’s time for me to change my plans.

If the sentence had not had “Creo que” and it was just “Es la hora de que…” would that mean the subjunctive would not be used in that case? Is “Creo que” acting as a trigger phrase for the subjunctive, or is more the general mood of the sentence that triggers it?

The trigger is the impersonal expression “es hora de que”. Creo que doesn’t usually trigger the subjunctive.

However, if you negate the sentence, that will trigger the subjunctive: “No creo que sea hora …”.