Conversion is of course linked to the way in which Antonio seemingly gets his revenge on Shylock at the end of the famous court scene in Act IV scene 1. Having almost had his pound of flesh extracted by Shylock, Antonio, reveling in the strange reversal of fortunes ushed in by Portia's legal loophole that she shares in just the nick of time, crushingly does everything he can to destroy his enemy, giving him a fate that is truly worse than death. Note what he asks the Duke to allow him to do:
Two things provided more: that for this favour
He presently become a Christian;
The other, that he do record a gift
Here in the court of all he dies possessed
Unto his son Lorenzo and his daughter.
It is important to remember that how a director chooses to play this scene will greatly impact what the play says about Antonio and Shylock, but one way of analysing these lines is to see them as a deliberate torture that is far worse than death. Antonio knows how intrinsic Shylock's faith is to him. To have to publicly recant and deny his faith and become a Christian is something that will be a torture far worse than any other punishment Antonio could devise. In the same way, to know that he will have willed his wealth on his daughter who eloped with a Christian and recanted her faith would have been a grevious blow to Shylock. The significance of conversion in this play therefore lies in the way that it is used to reveal the cruelty of Antonio as he gets his revenge on Shylock.