My employer has made payment arrangements with several vendors to pay down what he owes them. He would like to reclass the different amounts to Long Term debt from Accounts payable. Could this be doing by just debiting AP and crediting Long term? Do I still keep the vendor sub accounts as is? If I do then my AP aging and vendor accounts won't match. Or should I create Credit Memos for each vendor to clear out their vendor accounts? If so what would be my entry for the credit memo?

You are asking a hyperthetical here because you have not told us the term of the arrangements. It may be appropriate to open loan accounts for each creditor and to classify these at balance date according to arrangements made. If the arrangements will be dealt with before the next balance date no change is necessary