I'm going to employ "The Ledger Assumption Fallacy" people assume Van Wilder and say "He can't be Hal". Here are some points to consider.
1. Hal Jordan's appearance was based on the young look of Paul Newman. One of Newman's signature looks was his well defined facial features and toned body. Reynolds doesn't look like Newman but definitely has a well defined facial structure and body.
2. Hal Jordan is a no nonsense kind of guy who cracks a lot of jokes while doing what he does. Many of his Silver Age constructs were products of his sense of humor, egg beaters, boxing gloves, etc. Reynolds has shown he can do that and make it believable.
3. Geoff Johns who has been writing Hal for a decade has seen Ryan perform lines and has seen him in early costuming (not GL costume) and said on his Twitter that he IS Hal Jordan. That is enough for me.
4. Jordan is part of a larger picture, the GL Corps is the main star but Hal needs to stick out. In that abomination Wolverine: Origins, Deadpool had a small part where you could know its Reynolds and whenever he was on-screen he pulled attention away from even Jackman and that is what you need in Hal Jordan, someone who is carismatic enough to draw your attention even when there is a huge intergalactic threat or aliens standing beside him.
He'll be fine. The film will be fantastic. I have absolutely no doubt.