Replacing Dropdown-Field by Lookup-Field

Hello,

within my first steps with Zoho Creator I made a form with a dropdown field.
Now I'd like to change it to a lookup-field that gets its values from a different form/view.
But changing the fields type seems to be impossible (probably to keep data consistent?).
I would make sure that all values of the existing field (dropdown) would be part of the new possible seletion (lookup values) but anyway ... I can't change the type.

(btw: Starting over is not an option, because there's already too much data entered)

Any hints on that for me a a newbie with Zoho?

Thanks a lot

Jens