Rennie performed Klaatu with an air of benevolent omniscience. The audience sensed that Klaatu was kind, tolerant, and very wise. Reeves could possibly do the first two, but the second? I don't see it. I really don't.
Connelly is a great choice as she has the sharp intelligence that Neal has. Neal and Rennie had great chemistry because she could understand Klaatu's mission and its importance. Neal placed Klaatu's mission above everyone and everything else, including romance, despite intense pressure to become part of the mass hysteria.
In that, Neal was the first kick ass sci-fi heroine. She was the first woman in sci-fi to not just stand by and scream or, at most, possibly hit the alien over the head with a vase. She used instinct and intellect to perform the mission Klaatu gave her. Neal made her role believable and marvelously entertaining particularly since audiences had really never seen strong women in sci-fi films up until that time. I believe Connelly can do it, but she's stepping into some very big shoes.