I've also used a modified medium ALICE pack since I bought it in 1996, literally all over the country. I've been able to make it pretty comfy, but in agreement with others that any of the suggestions that @REI-JohnJ gave you will feel completely different, pretty much all for the better.
My questions to you are: What do you like about the ALICE pack? Do you have a bunch of ALICE/MOLLE pouches that you like and want to keep using them? Are you on a budget?
If you are okay with digital camo color, check out the ILBE. Used to be issued to the U.S. Marine Corps and based on the Arc'Teryx Bora 65. You can get the complete pack fairly cheap if you look around and it is about as bombproof as you can get. Tons of MOLLE webbing but it has a civilian fit and feel. Not the lightest pack, but several heavy duty handles make it easier than some packs to grab and chuck when you are portaging.
The earlier suggestions of renting a pack is a great idea, and absolutely get to an REI store if you can. Get measured and then try everything you can to get a better feel for what's out there and what you want.
Good luck, and hope you bring out ALICE every once in a while for old time's sake!