I measured this on a two-NIC 8.3 host.
As far as I understand, there's no way to do it from XO: the call that edits a network doesn't take a bond mode, every bondMode in the codebase sits on a create path, and the REST API on a running XOA offers create_bonded_network plus get and delete on a network, with nothing that edits one.
The CLI route looks cheap though. xe bond-set-mode on a live bond cost zero dropped packets in both directions I tried, pinging every 200ms across the change, and the command returned in about 1.4 seconds.
Two caveats, and the second is about your case specifically: creating and destroying the bond did interrupt the host (roughly 8 and 16 seconds), and I only went active-backup to balance-slb and back, never touching lacp, because my switch port isn't configured as a LAG and I'd have dropped the box.
So I can't tell you your lacp to active-backup move is free, only that mode changes in general didn't cost me anything.
Worth putting on https://feedback.vates.tech either way so the votes have somewhere to gather.