In a proxy re-signature scheme, a semi-trusted entity called proxy is capable of transforming Alice’s signature on a message m into Bob’s signature on m, with the help of some private information. In this paper, we have addressed the open problem of converting one type of signature into another, this was posed as an interesting issue by Ateniese and Honenberger in [1]. We propose a proxy re-signature scheme in which the proxy can convert a public key infrastructure (PKI) based signature of a user into an identity based (IDB) signature of the same user, we also propose an appropriate security model for the problem and prove the security of the scheme in random oracle model. In our scheme, the user provides some secret information to the proxy who is a semi-trusted entity facilitating the conversion process. The scheme we propose in this paper is single-use (the signature conversion is from PKI to IDB and not to another IDB or any other type of signature) and unidirectional (the signature conversion cannot be done from IDB to PKI). © 2012 IEEE.