Educate me on Self Propelleds

Hi, 

I'm looking into going towards the SP mower conditioner route.  There are not many people using them in my general area, and I need some eduation.  I did a search, but couldn't find my specific question.

 

I'm wondering on the New Holland Models, and the John Deere models...are the heads upgradeable?  In other words, will a new mower head fit on an older model SP unit?  I'm only looking at roll conditioners, not impellers.

 

I'm looking at a couple of models...

A New Holland HW345.

A John Deere 4990

 

thanks.

Educate me on Self Propelleds