It depends on whether you'd expect to get your investment back if/when you sell it, or just want to make improvements that would make it easier and more enjoyable for you to ride.

The frame and fork weigh around ten pounds and that won't change, but by changing to lighter and more modern components you can make the bike much more enjoyable. Typical upgrades would include swapping the heavy steel wheels for lighter aluminum alloy ones, ditching the kickstand, converting the crankset from the one piece steel "Ashtabula" crank to a lighter 3-piece alloy crankset, upgrading the derailleurs to higher quality, etc. These upgrades can be relatively inexpensive if you're a savvy shopper. You'll never get your money back if you sell it, but while it's yours it'll be a big improvement over the way it is now.

Browse around on the Classic & Vintage forum.