Changing gears on bullet 500 Cast iron

Hi all,

I have this weird problem all of a sudden. My babe has done 25K kms and has been properly run. Suddenly after the last Servicing ( nothing major was done), there is a terrible sound when i change my gears. Also, the smoothness that i changed the gears earlier, is missing.
Any reasons this could pop up suddenly? also, my mechanic suggests changing the clutch plate to 5 speed. is this good?
Please let me know.