Typically this will be OK.
I'm not sure about this statement:
"automatically populated from a calendar event sign up"
Are you intending to change the field type? It sounds like this is already a manual entry field but updated by a Pipeline or perhops a form rule.
As for changing a field to be a formula field of the same type, that will work fine. I suggest that you make a copy of that field first so as to preserve its data. When you copy a field it also copies its data. Just in case you realize you do need the original data for historical purposes or to use in the formula field for existing records created before a certain date (ie the date you made the change to have it be a formula field)
If you can be more specific about that second question I can help further.
Also be aware that you can easily copy a whole app to do a test.