What exactly do you mean by "rescan wallet"?


Based on this, it seems like you're trying to convert the wallet.dat to some other wallet format? If so, there is nothing that I am currently aware of that will do that.

At best, as jackg, suggested, you could export your private keys from the old wallet.dat and then import those to a new wallet. Bitcoin Core will allow you to export keys (even without the blockchain synced)... alternatively there are utilities like PyWallet that will enable you to dump the keys from the wallet.dat file.