Change to Apple One subscription

In my situation it was an annual Fitness Plus subscription, I confirmed with Apple support prior to switching to Apple One and was advised that I would receive a pro-rated refund for the remainder of that plan (which is exactly what occurred). If you want to be certain you can always contact them.

I believe I recall reading a reply from a forum contributor in that situation, that had the remainder restituted or otherwise compensated.

The public support documents do not describe this situation. For verification, you may want to contact the Apple Support staff. Contact Apple for support and service - Apple Support

Note that the Apple TV+ subscription gets canceled automatically when the Apple One bundle kicks in. No user action needed. There will be a one month overlap, as the first free month only applies to bundle components that you haven’t tried before.