you can't change plans, those options are only available for new customers.
I'm on the Photography ($9.99) plan and see options to switch to any of the other plans, and to pick the specific amount of network storage I want. Can't speak for someone currently on the all-apps plan, but it would be very odd for Adobe to have cut you off from the same functionality.
CC app in menu bar > user icon in upper right > Manage Account > Plans and Products/Manage Plan > Switch Plan. I see the two similar plans, but with drop-downs to choose any amount of storage ("conveniently" defaults to 10TB of storage at a hundred bucks a month!). To the right of the second option there is an arrow, which moves me over into the "All Apps" plans, all of which also have the same dropdown with the same options, defaulting to 10TB.
So I definitely can change plans. It would be silly for Adobe to not allow its current all-apps plan customers to similarly switch plans. If you really are not able to, call Adobe tech support, because that is almost 100% certainly a bug (why would they not want your extra money?)
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Maybe Adobe is being sensible here, they knew the all-apps plan users must be actual content creators, who should have no chance in hell trusting their whole catalog on the cloud.
You are not "entrusting your whole catalog to the cloud". You are backing it up (well, syncing it) to the cloud. If you are a professional whose livelihood depends on content you are creating, and are not backing your data up somewhere, IMHO you are not a very good "professional". As the old saying goes, nothing exists until it exists in three places - one of which is offsite.
That said, there are definitely better options for offsite backup out there, and Adobe's reliability as a cloud storage provider is completely untested, and its competency in keeping your files private in the face of increasingly sophisticated attacks, so in the end you are probably right that most people paying the all-apps monthly fee because they are making money from their content are not going to blindly trust Adobe with their offsite backups.