This really needs to be built, it doesn't make sense not to! Even if there is a huge difference in the way these account types work, there should be a simple process converting one to the other while keeping the same IBAN, keeping the same scheduled payments, and so on.
I have two 'buffer' accounts that I use for distributing income across accounts, so that 'there is always money in the bank for important payments'. One is a normal account, the other a savings account. Both work exactly the same, but I prefer the % in my dashboard so I'd like them both to be a savings account. This just shows me the status of my buffer, as 100% means I'm good to go for a month. (But I might change my preference later...) I don't want to change the IBAN everywhere everytime I switch, including in my own list of scheduled monthly distribution-payments.