I need to leave every three months to get a new Non-Immigrant B visa. Sometimes I do this just before the old re-entry permit expires. On return at Suvarnabumi, the immigration officers are often puzzled what to do, as I have a re-entry permit that is still valid for a few days and a new single entry visa. I have to explain that I want to cancel the old re-entry permit and use the new one. Sometimes they call a colleague/superior. Might take a minute or so, but it has never been a problem. In the end they just stamp the new one and cancel the old one.