The booking is easily amendable once in the DCS, which happens at around T-24 hours (or T-30 hours). Assuming you made the request within T-24 hours of your existing flight (not the one you wish to change to), there should be no issue at all. It's an easy process for the agent and there should be no "restrictions" to speak of.

I'd wager that the agent in this instance may have a very limited understanding of Astral.