What's the issue? You want to keep using the same field slug because you already have existing content that you want to now be recognised as a WYSIWYG field rather than multiline? Hence just creating a new field which implies a different slug isn't an option.
You can remove the multiline field from it Field Group(s), and then you can go to Toolset > Custom Fields > Post Field Control and locate the custom field and specify it is no longer managed by Types.
But that still leaves you with the problem that if you try to create a new WYSIWYG field it won't let you use the same slug, because it recognises the existing entries in the database.
So the only option I see would be to run a quick query to rename the entries in wp_postmeta so that they use a temporary different slug, create the WYSIWYG field with this slug in some field group, and then run a reverse query to revert the field slugs in wp_postmeta back to the original.
If you do that, recall that the postmeta key in wp_postmeta is the Types slug plus a wpcf- prefix (e.g. "wpcf-status" for a status custom field).
