No, once you create an account you cannot change its type. If you accidentally set up an account with the wrong type, simply create a new account with the correct type, then copy and paste all the transactions from the old account into the new one.
If you are handy with Applescript, you can use a script to change an account's type. Download some sample scripts from our website to get started, but please be aware that any modifications made your data file with Applescript are done at your own risk.