Convert from iCloud to Apple One - do I have to re-upload?

Thanks Foxfifth. Yes, the iMac "optimise" option is always turned off. So I signed up for Apple One Premier and it all seems to have worked as promised - even including refunding unused portions of the existing subscriptions.

The next challenge is to safely move my partner's iCloud backup storage from her own separate 200GB account, to the 2TB in Apple One Premier, as a family member, so that we can close her existing iCloud account. I cannot find any guidance in Apple Support articles on how to do this. Is there a step by step guide anywhere? My life is forfeit if I lose her backup ...

This doesn't seem to work quite the way it is described in the article linked above. I have set up Apple One Premier with me as the Organiser and I finally got round to switching my partner's iCloud storage to Family. The article says:

1. Choose System Preferences, click Family Sharing
2. Click iCloud Storage.
3. Click Use Family Storage.

But on her iMac, when I get to step 3 there is no "Use Family Storage" option. Instead there is a paragraph encouraging me to share her 200GB with family, and there is a button labeled "Share". We don't want to share her 200GB, we want her iCloud storage to be part of the 2TB Apple One Family storage. Am I missing something?

Not exactly.

I was Family Sharing Apple Music before I signed up for Apple One, and without doing anything else, Music, Arcade, Fitness and TV all became shared with my partner, so that when I looked at Family Sharing on her iMac, all those services were available to her as an Adult in the Family of which I am the Organiser. As expected.

After seeing your last post just now, I went into Family Sharing > iCloud Storage on my iMac and turned it on. So it looks like what I have posted here, which is what I would expect - me using 241GB and her not using it yet. Problem solved I thought ...

But when I look at Family Sharing > iCloud on her iMac, this is what I see. Which is not what I expected. There is still no option to choose to use the 2TB of the Family subscription. If I click Change - I just get the option to buy 2GB in her name. It is weird because on her iMac it shows that I am using 241GB, and that she is not using the 2TB, but it still says she has 200GB available with no option to join the Family 2TB plan. Is it possible that her iMac already had iCloud sharing turned on, but with no-one invited?

Thanks. Did that, first with chat then escalated to phone rep. Neither of whom could solve the problem as it was presenting on my partner's iMac.

So I did what I should have done earlier and tried the alternative route, following the instructions in Share iCloud+ with your family - Apple Support for iPhone or iPad. And everything worked as it should on her iPad. Once she had elected to "Use Family Storage" on the iPad, it then appeared correctly in Family Sharing > iCloud on her iMac.

Seems like there is a bug in the way this is handled in macOS Monterey. My iMac is an old Intel 27" stuck on Catalina, so maybe not seeing the bug.

Thanks Foxfifth, you being Level 10 in this forum, I am going to trust you on this and go ahead. After zealously backing up the iMac on 2 separate disks ...

Thanks, apart from your explanation, the bit I needed and hadn't stumbled on was in the linked article - ie where and how she elects to share the family storage. We will do that now.