Thanks for the good question!
People need to remove the old/existing Appcoin-folder from the %appdata%/roaming-directory and after that, launch the new Appcoin Client which will be available after Appcoin launch on Sunday, March 9th, 18:00 GMT in OP of this topic + at our websites http://www.appcoin.info/downloads/.
The old client(s), folders and wallet/coins should be deleted since they will have no use anymore.