I took my old Coleman Peak 1 external frame backpack pack and took the old sack off and replaced it with some feed sacks. You can see some pics of it here, in this thread.

Admittedly, the material I used is pretty hobo, you can probably find something off the shelf that's better than I used, but the design has some real benefits, and I didn't have to alter the original sack or the frame to do the mods so nothing was lost in trying.

The pros are:

• Adjustable storage space. (Both more and less than the stock sack)
• Lighter weight. (You could lose as much as a pound)
• When packed, the sack is compressed and shifts around less.
• The sack is vertically adjustable on the frame.
  
• The load is better distributed and easier to carry.

The cons are:

• All those straps that hold and compress the sack take a bit of fiddling.
  
• It's a top loader design, so you have to use a lot of ditty bags to keep your stuff organized.
  

Any way, I think those old frames may have more life in them yet. Just strap a UL sack on them and they'll feel brand new!