It sounded to me like you didn't backup your wallet.dat while upgrading (or didn't put the backed up file into the %appdata% folder before starting mint). If you have your wallet.dat anywhere on your computer (even if it's very old), you will be able to recover your coins. Close your wallet, copy your old wallet.dat into mintcoin's appdata folder, open mint, if you see your addresses but not coins, try "checkwallet".


If you got your coins from mintpal or poloniex, those exchanges are guaranteed to be on the correct chain, so you should have your coins in your address no matter what (even if you were on the incorrect chain while at 1.5, as sender was on the correct chain, the transactions would have been accepted in both chains). Still if you don't have your old wallet.dat, you cannot claim them