The answer to your question about why Hal suddenly seemed older in Green Lantern #1 than he had been previously in the Action Comics Weekly Issues can be really be traced back to Mike Grell's work on the Green Arrow series.
Following the Crisis when the DC titles were being rebooted, (IE: Man of Steel and Batman Year I), Mike Grell had begun working on the new Green Arrow series. One of the major points of Grell's Oliver Queen, was that this was an older hero, not a new one. Grell's Queen had been around the block quite a few times, in fact it was became an annual event in the book to see Black Canary (Dinah Lance) and Ollie celebrate Ollie's birthday.
I don't have my GA books in front of me, but I believe Ollie is shown to be in his 40's.
Now, following Crisis, Green Lantern did not reboot. The series continued on exactly as it had before. Indeed, both the "Legends" and "Millenium" events take place in an un-rebooted Green Lantern book. I could be wrong on this, but I think that Green Lantern was the only major title not to undergo a reboot.
Later, when the title was eventually cancelled, Hal Jordan inherited the lead role in Action Comics Weekly. But after some time, DC decided to re-tell Hal's origin and we were given the "Emerald Dawn" mini-series. We were now ready for the new Green Lantern #1.
Of course, a new Green Lantern series presented somewhat of a problem. Since, the character of Oliver Queen had played more of an important role in Hal Jordan's life than any other single character, a decision had to be made whether to trash that aspect of the character or not. Obviously, you couldn't just reboot Hal and have him be young, while Oliver was over in his series being an elder hero. Also, at that time, Green Arrow was selling very well, and it seemed as though the concept of a title dealing with an elder hero would play well in the marketplace.
So, it was decided that the Hal Jordan in the new Green Lantern series would be an older Hal Jordan than the one we had just seen in the Action Comics Weekly books and the two Green Lantern specials. I believe that one of the first lines in Green Lantern #1 is "Its been 15 years since I first put on the ring." The graying temples were done to make an impact on the reader that this was not the Hal Jordan you had been reading earlier. And I have to tell you, those first eight issues of the new GL were an incredible read.
I hope this clears up any questions you might have on the subject. In brief, there was no explanation as to why Hal was suddenly old. He hadn't vanished or anything. One day in Green Lantern Special #2 he didn't have the gray and then suddenly in Green Lantern # 1 he did. The gray was done simply for the impact of showing that this was an older Hal Jordan, one who had been a hero for 15 years... It is as simple as that.
Of course I should point out that with DC's new mandatory 10 year time line, it is impossible that Hal was GL for 15 years and therefore the gray no longer makes any sense.. Both Hal and Oliver Queen were de-aged following Grell's departure from the GA title. I gather than they continued to show Hal with the gray because Marz and Banks had forgotten the reason why Hal had been drawn with them in the first place.
If you have any further questions, do not hesitate to ask!
-=Marc=-