The new Imperial Guard Codex, while adressing alot of the popularity problems of the old guard, pretty much stranded those of us with old Imperial Guard armies.
The first thing I would tell you to do is pretty simple. Print out my codex, and use it instead. It allows you to use any of your old troops under the new system. This still leaves the problem of your heavy weapon squads, but other than that you should be just fine.
You can also just forgo the whole new list, and use the old one. Somewhere in the pile of White Dwarfs, they outright say, if you don't like the new lists, you can use the old Codex lists from the boxed set. And from what I understand, this goes for tournaments as well. Unfortunately, you loose all the cool new stuff the IG get, like weapons teams and whatnot.
Throw out all your mini's. I've seen alot of people doing this last one. Thats just insane in my opinion. Either a) change them around a little to fit in better, or b) just start a new army. Yike.
Assuming you are willing to butcher your old squads a bit, here is how you can make it all integrate more or less seemlessly.
It bent me out of shape that GW decided to butcher the heavy weapons squads. I used to like the 5 weapons per squad. And of course, it makes a mess when you try to divide 5 by 3.
The way I converted my squads was pretty simple. First, I picked an old tactical squad, read them thier last rites, and gave them the big axe. Using my old collections of arms and weapons, I was able to ressurect a few members. Start by re-organizing all your heavy weapons squads into groups of three. Most likely you will have a spot or two left over. Simply convert members of the tactical squad to fill in these spots. At that point, if you have a few members of the old squad left over, simply rework them into either another weapons team, or a command squad (with a little work obviously).
Unfortunately, all the old IG tanks got the big boot, and they can now only field new tanks. You can either repaint these for another army if you collect one, or try to use my codex. There isn't much that will make a predator into a chimera magically.
Yes, yes, I hear you. They pulled your nice medic out, and axed the cool standard bearer. If you really want to stay true to the spirit, you can shave the medical icons off the medic, and use him as a comm-link figure. The standard bearer can probably be left alone. Or you can just pretend they are weapons crews. If you don't mind using my codex, you can replace the medics with troops from your butchered tactical squad, and use them in the Imperial Armor Codex. The standard bearer can be used as is, but you may have to migrate him into a command squad without heavy weapons.
Hopefully this will help those of you still stuck in the migration, if not, too bad.