This appears to be true today, and unchanged with the existing beta of Email from Anywhere:

If you’re creating an email assist from a CTA once you’ve selected a template, if you save that email as a draft either directly, or previewing the email and closing the task so it ‘auto-saves’ you can no longer change which template you have selected.

Yes, you can still edit the email but if for whatever reason you need to change that template you have to delete your task and start a new one.

Please allow end users to change templates any time before they send an email, and not just lock users into ‘Validate Email’ which blocks them from adjusting anything, if they’re working off a draft.