Is it possible to change a group customer type all at one time or do I need to change one at a time?
You can do an advanced customer search for all of the customers in your database who are tagged to the customer type you want to change from, then export those results into a spreadsheet. Then you can change the customer type for all of those customers in the spreadsheet, taking care to ensure that the new customer type is already created in your database, and that you spell it exactly the same way (including spaces, capitals, etc) on the spreadsheet. It must be an exact match. Then you can upload that spreadsheet and it will change all those customers' customer types at one time.