OK sports fans, I'm gonna give you some notes on rucksack mods. Depending on what you need, there's lots to choose from. I went with a LBT 2657 cuz I wanted something in OCP to put on a 1606 frame. Still plenty of Med and Large ALICE bags out there. Large Molle, Molle 4,000, not to mention the FILBE. I will give a generic description of breaking down the LBT, but it pretty much describes all of them.
First of all decide which frame you want. I started out with a DEI 1606 but ended up with a Crossfire DG-16. I would pretty much stick with either of these frames over metal. Whichever you chose will determine where the "tabs" you add are going to be placed. If you do chose a 1606, and set the bag up for that, you can upgrade later to the DG-16, as the tab pattern will still fit.
Now you are ready for the tear down. At the very least you will have to open up the main side seams to insert frame tabs. If you pick the DG-16 you also have to decide whether to keep the top sleeve, or go with the Crossfire style "jockstrap". I like this arrangement better because it gives you total access to the top of the frame, which allows you to put your top load stabilizer straps exactly where needed. With a top sleeve you can't use to two most outboard positions, which is what I prefer.
To open up a ruck. It looks like a huge task. And it may be but if you take it one step at a time, it is doable. I use a no. 11 x-acto knife. Yes it can be risky, but with a little practice I find it is much quicker and easier than a seam ripper. But just like the Boy Scouts (used to) teach you, keep your holding hand in back of the cutting hand. That little bitch is sharp. Put tension on the seam, until you see stitches, and careful pick them with the blade. Once you make an entry, choke up on the material, keeping taunt, and continue to rip the seam.
With this particular ruck, you remove the top flap. Then you remove the "storm" collar, which gives you access to the side seams. With an ALICE you go right to the side seams. If the seam is taped, you find an entry, and work the tape off first. I will work both stitch lines, back and forth, as I go. Once the tape is off, you go to work on the seams themselves. Be careful where webbing is inserted. The material will tear here because it is sucked in so tight to the webbing. Be sure and pull tension so the blade stays on the thread only.
At this point, you are probably wondering if you can get away with just opening up the seams enough to get the frame tabs in. The answer is yes, but I will esplain why I don't just do that. Over the years I have found that it's just easier to break the damn thing down, into component parts, work my magic on them, and then re-assemble the damn thing. Trying to do something, with the rest of the ruck hanging onto it, moving all that crap around to sew something in, well it's just a lot harder IMHO, than just opening it up all the way up. But this is an individual technique; you gain experience, may find you like to do it another way.
A note: I work on hard wood floors because it's easier to sweep up all the thread fuzz. It makes a fucking mess. If you are on carpet, I would advise you to lay some sheet plastic down (or newspaper), under your work area. When you are down on your hands and knees, trying to vacuum all those little fuckers out of the carpet, don't say I didn't warn you.
Ok at this point, we have taken the seams completely apart. This usually the point where my wife comes in and says "Is that the new pack you just bought!?" Just flash you best "gay adventurer" smile and say "Yeah, no problem."
Now we are gonna clean the parts. Get some small curved and straight hemos. These are the best tools for picking all that cut thread out of the parts. It's kind a PITA but you'll get the hang of it. Work over your trashcan as much as possible. I use a beer case box, lined with a plastic bag, which gives you a nice wide mouth. Work one stitch line at a time, top to bottom, left to right; find a pattern that works for you to systematically remove all that crap.
Once the thread is completely gone, examine the base material. Any small rips or tears can be repaired with McNett Seam Grip. Dab a little on, let it set, shape it with your fingers, and let dry completely. If you have a big tear, you'll have to repair it. I'll cover that later. Now you may notice you have all these little fine holes in the material. What I do, is lay out the part on a flat surface, like your lay out table, and then "rake" the holes with the blunt nose of the hemos. What you're trying to do, is move the individual fibers around, and minimize the punctures. This sometimes takes several passes, on both sides, to clean up. Some materials work better than others, but in general this will close up the holes enough to be cosmetically if not structurally repaired. This may sound a little goofy, but it works. Also good technique for just regular sewing, when you take something out and want to re-do it. If you leave the perforations, the needle will want to track in the old holes, again, instead of where you want it to go.
Holy shit, the piece is ready for re-work. Now we gotta figure out what new stuff we want to add.