It’s not possible at the moment to convert Text field to either Single Select or Multi Select. As a workaround you could create a new field and then select all Text values, then copy them into your Single Select or Multi Select field. Without this feature it’s very onerous to “change text fields into Select fields”.
I believe that this used to work.
I haven’t checked how it works, but is the option to allow non-creators to create new options enabled?
Nope 
I’ve added a Feature, but that’s the first request so far.
I wonder: does the Text Field come from an integration?
Thanks. Correct - it comes from Notion integration.
And in Notion the Field in question is a Select, right?
If yes, I’ll add your +1 to a slightly more popular request to support Selects in integrations.
Just to clarify 
- there is Single Select and Multi Select. In my scenario I needed Single Select for one field and Multi Select for the other.
Heh, looks like I hit exactly that problem now trying to create a sync between one of our Google Sheets and Fibery database using Integromat, so definite +1 from me 
I’m pretty sure it works now - just tested, am I going crazy or have you changed this?