I looked in your laptops manual here
http://www.helpdrivers.com/english/download/
I dont see anything that says you can do it.. however, a small downgrade shouldn't sabotage your laptop. I am unsure why you would downgrade since a reason wasn't provide but to safetly answer your question, if the GT635M is an actual card, then attempt to replace at your own risk. I dont think it would do any damage, however, I would invest in the replacement instead of downgrading.