Do people who have been baptised Roman Catholic have to be re-baptised Greek Orthodox if they convert?

Despite what some will tell you, to be baptised is to be baptised a Christian and not a Catholic, Anglican, Baptist, Orthodox or any other denomination. Provided the baptism is done in the name of the Father, Son and Holy Spirit, as commanded by our Lord at the end of Matthew's gospel, then, the baptism is valid whatever denomination. There are some hard-liners that will argue that you are baptised a Catholic only, or an Orthodox only, or unless the baptism is done by full immersion it is somehow not 'valid'. However, the key factor in baptism is the content of someone's heart (either directly or by proxy as in the case of godparents in infant baptism) and not by some man-made denomination system. So a baptism as a member of the Christian Church worldwide, in the name of the Father, Son and Holy Spirit, is all that is required in the sight of God.