I am unable to change a project's budget type from Retainer to anything else. Why?
There are two situations in which you cannot change a retainer project's budget type.
The first situation is when you have locked time periods on your project. This does not allow you to change its budget type. What you can do here, is just unlock those time periods and you should be able to change the budget type.
The second situation is when you have invoices already created for this project. The only workaround for this situation is to change the status of all of the project's invoices to Draft and then try to change the budget type. However, if you have an integration for invoices, you might first need to unlink the invoice and then change its status.