Yes, you can change the rear pulley from a 70-tooth to a 65-tooth without changing the belt, but it is not recommended. When you change the number of teeth on the rear pulley, you are changing the overall gear ratio of the bike. This means that the bike will accelerate more quickly with a smaller rear pulley, but the top speed will be reduced. Additionally, the bike may run at a higher RPM which can cause the engine to overheat.
It is generally recommended that you change the belt when you change the rear pulley to ensure that the belt is the correct length and tension for the new pulley.
