Well you'll need to start working with them while they are penned before you let them out... but the main key to re-programming adult birds to a new coop/run/home really is keeping them penned long enough for them to forget their old routine and habits, so try to stick with the 6 wks if you can. It's a good idea to always make the same call over and over when you feed/water/treat your birds, so they learn that call means food, and they get used to you and know that you are the food goddess... lol. You want them happy to see you, and meeting you at the gate to see what you are bringing them.
When I finally let my birds out I start with just short outings at first, in the late afternoons/early evenings (while I am out there babysitting them so they don't wander too far) and I herd them back in/call them in for food when it's the feeding time/coop-up time (you may need a calm helper for a while). It helps to let them out hungry, so I start taking away their feed around noon (if I am going to let them out that day), that way they want to come back in and eat (I also shut the pen after they've wandered out, so they can't come and go as they please and eat all day). I make a routine of all this and try to consistently get them in at the same time each evening for a while (when the sun is going down helps visually cue their little brains that it's time for bed), until they learn they have to go in each night, no ifs ands or buts about it. Once you have a routine established with them it won't take so much time and energy, but it's best if YOU establish the routine for them, instead of letting them establish their own (like roosting in the trees). Like Pop mentioned, it might help if you only let a couple out at a time at first, and leave a few in the coop/pen so the loose birds want to stay close. Some have pretty good luck with that method. If it's a fiasco the first time you let them out, you may want to wait a couple more weeks before trying again...
Guineas wander, they don't always stick close to home, so unless you are babysitting them/herding them back to the coop/run area the entire time they are out they will most likely cover the 40 acres all on their own, eventually (and maybe even farther, lol).