I've tried performed the below steps and I didn't see any issues, so i think you can go ahead and change the column datatype:
- Created the table with the column type as unique identifier.
- Created the stored procedure to insert a value into that table through NewID() function.
- Execute the stored procedure, and data was inserted without any issue.
- Now, I changed the column type to varchar and again executed the stored procedure.
- Procedure ran fine without any issue, and data was inserted.
So, the answer is Yes, you can change the column data type.