The Quick Answer
In short, you will only see a "Switch to Open Banking" on Desktop and "Migrate account" on Mobile if your bank provider has enabled Open Banking API connections.
So when will I get to switch to Open Banking?
At the moment, only the nine largest banks are required to implement the Open Banking API and they are phasing the roll out of connected accounts. This means you might still need to use an authorised credential sharing connection (like the way you currently connect your accounts in Money Dashboard).
We will continue to provide connections to as many accounts as possible, opening up new Open Banking API connections as soon as they are available.
But my provider does enable Open Banking API and the option to switch is still missing
Not all account types fall under the PSD2 (the legislation requiring banks to make your account information more accessible to you) so you might not be able to migrate all your accounts to an Open Banking API just yet. For example, most savings accounts don't.
With savings accounts a good rule of thumb as to whether your bank will provide an Open Banking API connection is if you can currently make payments directly from your savings account to another third party account. Note, ISAs are not currently available to connect via Open Banking.
Other useful articles on Open Banking