Prestige cannot replace Executive because it no longer provides any Admirals Club access, but you might want both. While Prestige is nominally $450 it actually costs $200 because there is a $250 airline credit. (It's even less if you have a Citigold banking relationship.)
Prestige would add Priority Pass lounge access and a discount on hotel stays of 4 nights or longer:
http://www.flyertalk.com/forum/citi-...embership.html
http://www.flyertalk.com/forum/citi-...ly-2017-a.html