Asterisk doesn’t rewrite SDP; rather it creates a complete new set of SDP on the outgoing leg based on what it selected on the incoming leg, and what is configured for the outgoing leg endpoint.
I suspect the restriction in Joshua’s reply to another thread:
still applies.
Note you didn’t say which version, and a full parameter dump is difficult to interpret, because one has to consider what is already default. If using config files, which I would always suggest if you are experimenting, having just the options you are setting is much easier on the brain.