We work with Palmetto as our MAC and they don't allow "normal" corrected claims using 7 on the claim to show correction. We have to go thru the Palmetto eservices website and do the "simple reopening" of a claim or call customer service and do the reopening over the phone. They will let you add/remove/change modifiers, change diagnosis codes, simple corrections that need to be made. If it's more involved and can't be done, they usually let us know.
Hope this helps.
Jennifer