I think they put things up like that so ppl don’t abuse the system, and trust me ppl do abuse the system with deliberate multiple SIRs. But they always tell you if you are awaiting an appeal to always SIR somewhere in case it doesn’t go through, and then if you get in, to cancel the previous one.
Trust me you can withdraw and SIR somewhere else.You can SIR while another is already in place, although they don’t especially like it.
So OP, when you figure it out just go change it. If you feel more comfortable calling someone to verify, certainly do that, but you don’t have to.
/----
Here is the wording from UCSD:
"If you’ve changed your mind and want to decline the offer of admission from UC San Diego after you’ve already accepted, you can log in to MyApplication and click the Cancel My Admission link on the MyApplication checklist.
Or send an email to sirdecline@ad.ucsd.edu. Be sure to include your name, application ID number, and a statement telling us that you are declining the offer of admission."