Thank you very much for your reply.
Well, I'll describe what happened and the outcome of the entire process in case someone needs to go through the same process.
I contacted Apple Support, Voice Support, and explained the reason for canceling AppleCare One and switching to AppleCare+ with Theft and Loss Protection Annual Plan.
Reason: I wasn't able to add some Apple products I own to AppleCare One because they were no longer eligible, so AppleCare One wasn't worth it for me.
Concern: The only device I could still add to AppleCare+ was the iPad Pro M4, but I only had 20 days to enroll. My concern was whether, once AppleCare+ One was canceled, the device would still be eligible for the AppleCare+ with Theft and Loss Protection Annual Plan.
And... Yes, the device is still eligible, and it's possible to switch plans.
Note: When I canceled, I waited around 30 minutes and was able to purchase AppleCare+ with theft and loss protection annual plan.