I don't see why they need to mess with his costume. They haven't dramatically changed Superman's costume in the films. He still wore tights in Superman Returns. Why can't Hal?
This isn't exactly true. When the Christopher Reeve film was done, the blue was made brighter to show up better on screen, the shield was enlarged dramatically and the "S" was changed completely. Not to mention that the "S" on the back of the cape wasn't a staple of the comics at the time. Many hardcore fans were outraged when they saw the Superman film costume, the thing is for us its just how it always has been. The same was done with Superman Returns where Singer muted the color scheme and returned to the royal blue tint but then coupled it with a burgundy hue to the red, created the smaller "spankies" to make his chest look elongated and to make Superman look taller while the Reeve costume had the bigger spankies to make the chest look more barreled due to the smaller torso spacing. He also changed the boot structure and embossed the "House of EL" crest all over the thing not to mention returning to the smaller shield of the Golden/Silver age. Again fans were outraged.
Superman, Spider-Man, Daredevil, The Fantastic Four all had costume changes that could be considered dramatic with just keeping close enough ties to make it recognizable. If you look the conceptuals though there were severely dramatic changes pitched that made to the final decision realms.
The Green Lantern costume will have changes done that will probably mix elements of al the most notable mainstream GL Costumes including the original, the animated looks of Hal Jordan and John Stewart, the Alex Ross throwbacks, the new "resurrected" costuming and I wouldn't be surprised if they didn't throw a bit of Parallax in there.
The positive thing is the screenwriter wanted to keep the original colorscheme including the white gloves and wrote references into it but the rumors are that the classic look will be Abin Sur, similar to this:
Although recently they included this into his look taking some queues from Guy Gardner (It was an elseworlds though):
and have Hal create his own look once he gets control of the ring.
This is an official conceptual:
Released by the studios similar to when Marvel Studios released Phil Saunders' conceptuals and even Adi Granov's conceptuals for Iron Man so I wouldn't expect that above but you get what they were going for.
One rumor that was going for a long time which hasn't really been debunked is that they are looking at Kyle's newest costume as inspiration similar to when the Grayson Robin got the Tim Drake Robin costume in Batman Forever
Which is evident from the recent "First Flight" Hal look:
Another is that they would be going for Emerald armor, meaning that the Green portions would be actual emerald armor over the black suit, which hasn't been debunked either which would look something like this:
We've got plenty of time of looking at different costumings. What real die-hard GL fans are looking at isn't the look of the costume but more the look of the film ring. Each GL since Hal has had their own ring design. Alan Scott's ring was individual and then in Hal's time they gave a uniform ring look like Lucas' Jedi Lightsabers and then with the introduction of Gardner, Stewart and Rayner the rings began to take individual looks. All three started with the standards and now they've all got their own looks, even the Corps members were given individual looking rings. They've already changed Hal's ring for First Flight:
and people are worried that they'll give him Kyle's look which is just the large Lantern look or John Stewart's look where its the classic look but the Lantern symbol is raised over the flat design. Hal's ring has always been either completely flat or have a recessed "inner circle" where the lantern isn't any taller than the outer circle but at the same height. The First Flight ring is completely new, breaking the mold for any Jordan rings and for a lot of fans looks too much like Kyle's ring than Hal's.