Within AccessAlly they can update their Ontraport credit cards, and of course change their AccessAlly password too.
Cancel subscriptions inside AccessAlly by applying a tag to then trigger a “stop subscription” event in a campaign in Ontraport. And for buying new courses, the order forms can continue to be Ontraport forms and it can be link within AccessAlly to purchase more/those. 
So basically what you described is totally doable, it’s just a matter of setting up some accompanying campaigns in Ontraport and the right pages on AccessAlly: