I've done one "rebuild" at a local transmission shop that cost $2000, and a full remanufactured warranty replacement which was $3500. Might be worth it to shop around as rebuilding a 4L60 is a pretty common thing and lots of shops do it.
Replace with a 4L80? Well, with enough money, time and effort, anything is possible. The 4L80 is physically longer than the 4L60 so replacing a 4L60 with a 4L80 means moving the transfer case back, shortening the rear driveshaft and lengthening the front. It seems to me that if you're going to spend that much money, why not just look for a 2500 in the first place? GMT800 2500 Suburbans are not nearly as common as 1500s (I'd estimate there is 1 2500 for every 20 1500's available on the market) but they are out there. IN the GMT400 models, 2500's are a little easier to find, probably more like 1 in 10.