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What software version is the 3504 running? You need 8.2.110.0 or later for a 3800 series AP. https://www.cisco.com/c/en/us/td/docs/wireless/compatibility/matrix/compatibility-matrix.html#pgfId-590411
I know this is a few months old but just wanted to say - you could probably do it with EEM:https://community.cisco.com/t5/wireless-mobility-documents/automated-backup-ssid-with-eem-on-catalyst-9800-wireless/ta-p/3743838
