sweeny

Is it possible to change the freewheel on an older road bike to a cassette? I have a SunTour Perfect 5 cog freewheel and i'd like to upgrade to something with more gears because i'm moving to the mountains. I know i'd have to change the hub, but i need to anyway. I guess what i'm getting at is, will it work? I'd like to replace the entire drivetrain in the meantime aswell, should i go ahead and switch to cassette? can i do that?

Retro Grouch 

The short answer is: Yes, it can be done.

A better answer would be: I'ts not likely to make economic sense. As a general rule, the cheapest way to buy bicycle components is already bolted onto a frame. By the time you change out the whole drive train of your bike, you will probably have spent quite a bit more than a different bike would have cost you. Old, lightly used road bikes are pretty much a dime a dozen. Find one that's the right size and that's set up the way you want and buy it.

halfspeed

You could go with a seven speed freewheel and keep your hub. Are you sure it needs replacing and not just an overhaul? If you then add a triple, you'll have a 21 speed bike which should be plenty and may not require upgrading anything else.

Seven speed freewheel: ~$20
Sugino XD300 crankset: ~$50
Shimano UN53 bottom bracket (may not be necessary): <$20
New derailleurs (may not be necessary): ~$100

If you've got down tube shifters, keep them. Upgrading to anything indexed is probably going to push the price up to where a new bike starts to make more sense, although barcons are not out of the question. How much do you love this bike? If it's a great frame (high quality double butted steel like Reynolds 531) that fits you perfectly and is in good condition, it might make sense to modernize it. In that case, you can get new hubs, have the rear triangle respaced, rebuild the wheels, and go with an eight or nine speed drive train. I'd still be cheap, though and go with barcons or DTs over STIs since you already have brake levers.