How can I convert my under cabinet lighting from halogen to LED?
Your G8s are probably 120VAC, right ?  As such, you are not going to replace with LED without switching to LOW DC voltage.  Most are 24V.  

If you want dimmable LED (a good feature), you'll need a dimmable constant current driver and a magnetic transformer and a magnetic rated dimmer.  You can use a  standard dimmer, but the current surge will burn it out. (ask me why I know...)

Bottom line.. rip out the sockets.