Uhh so OldTrees basically said the same thing I did, and you agreed with him...
Well, he was more explicit. If you meant the same thing, then I agree with you too.
So then you retract your statement that " changes in G must be offset by an opposite change in C+I." ?
Let me think this through. In a transaction, when government spends, the government bank account is marked down and the recipient's (i.e, private) bank account is marked up by exactly the same amount. When the government taxes, the payer's (private) bank account is marked down, and the government's bank account is marked up by exactly the same amount. So I guess what you and OldTrees said exists as a possible effect over time (i.e., a stimulus), not an instant case of account bookkeeping.
The government does have the power to mark up its bank account without collecting taxes (or selling bonds). That's often called "printing money," even though most money these days takes the form of electronic records. However, US law, and more importantly its political environment, makes this difficult to do.