Needing some input...

My gs forks are needing rebuilt. Rotor is in despair. Caliper needs rebuilt maybe new pistons....

I have my 1st bike parked (85 Honda sabre 700)  It needs welding on the frame and tons of tlc  i cant give right now.. but I had rebuilt its forks before frame snapped. It has 37mm forks but are a little longer. 

What sort of issues could I be looking at swapping the tubes down including wheel brakes and such.  I know to measure  the tube height. And I can swap the master cylinder as its dual rotor.  The sabre is a heavier and faster bike so im thinking it wouldn't be an issue ? But idk ive never done this before

It's very unlikely the tube spacing is identical, so you won't be able to just swap the whole set wheel and all. Even if you fabricate spacers to make the wheel fit, the brake rotors probably won't line up with the calipers and you may even run into real clearance issues, like the rotor touches the fork tube, or if you mill the wheel to move the rotors, then the calipers touch something like the rotor mounting bolts or something.

OF course, you could swap the triples from the sabre and fix all of this, bit you'll have to deal with getting the axle and bearing surfaces to match, which usually would mean taking the gs steering axle and pressing it into the vfr triple and doing whatever machining is necessary to make it work.

Just rebuild the gs forks. It's 5% of the effort and 1% of the risk.

Agreed....just rebuild the GS forks. You would likely have to swap the entire front end to make your idea work which would involve custom spacers/bearings for the triple tree at best and possible a whole custom triple.

All those rebuild parts for the GS are cheap, except for the rotor. But good news! You don't have to buy any of it new. Just get used parts off ebay. There are entire GS front ends on eBay for less than the cost of a new rotor. Or rebuild the forks and caliper and just a get a good condition used rotor. You can buy them all day on ebay for $20 - $40. The rotors last forever and its easy to find a use one from a relatively low mileage bike.

"Rebuilding" GS forks is just $30 for seals and oil. Similar cost to rebuild the caliper. So you could have a fresh front end for under $100 and much less effort than the swap.

Have rebuilt the forks and went with progressive springs .... big mistake. Rotor is getting pretty bad. So I'm looking at a good 200$ just to rebuild again. And still have suspension  that will disappoint.  Sonic springs aren't a good choice for me either as half the time I ride 2 up half the time I dont unless I change the springs frequently 

Fork tubes are 4mm more apart on the sabre.  Looking into pressing the gs stem into the sabre triple tree. 

I have .90 sonic springs and find them perfect because you never ride as aggressively with a passenger as you do on your own. I rode Oregon to Arizona and back with a passenger on my GS and never once thought about the suspension. Did another trip a few years later, Oregon to Mexico and back on the same suspension by myself and the suspension with Sonic springs was great for that too. Sonic or Racetech tuned to your weight and riding style is the way to go. If you really need to spend more money, the Racetech valve emulators are good, I have them on another bike and its nice to be able to adjust compression and rebound damping separately.  Progressive springs are a one-size-fits-all bandaid and a marketing gimmick in my opinion since all forks are "progressive" since the air in the fork has progressive resistance as it is compressed.

I doubt the technology in a '85 Saber 700 fork is any better than the GS, so it doesn't make sense to go through all the trouble of the swap and then still have to tune the fork when you are done.

progressive springs work well in my Triumph simply because it has inadequate suspension travel. To prevent bottoming, you must choose a spring that's too stiff to respond to ordinary road bumps. So the progressive springs are a workable compromise.

The GS has about an inch more travel and is 100 lb lighter which makes a world of difference. It can run straight-rate springs tuned correctly without risk of bottoming under normal circumstances. However, if you do switch between single rider and 2-up routinely, and your pillion is not very light weight, then progressive springs may be a good compromise, since they stiffen under preload.

FYI it's easy to cut the progressive springs, cut some of the soft coils off and it will get stiffer at the top, more quickly run out of the soft portion. You need a dremel or angle grinder and a bench grinder to do it, and clamp a vise grip on the spring coil behind where you are working on it with the grinder to keep the heat from spreading too far.