How do I switch a return filed as "Married filing Individually" to "Married Filing Jointly"?

All you have to do is file a form 1040X changing your status to mfj. On the form you can put down the amount of the refund your wife has received already. With the form 1040X you should have your wife sign and send a Form 8379 "Injured Spouse Allocation" which prevents the IRS from taking any of her refund to satisfy a debt that you owe.