Help with Many to Many Table Data Entry
Even if you have a UNIQUE constraint on username.
You must replace all referenced Columns with the username and change the dataatype of column.
A text field is also usually slower that an Integer.
So when you see all negative points, there is no good reason to change it now.
Besides, what does it really matter, if you have a numeric id that you have to use to identify the user or another unique text filed.