Well, I didn't have to get used to them, it's just what we had back then. Now that I build bikes a lot I don't know how people get used to building indexed bikes. It's way easier to learn to shift friction than it is to get an indexed system working right.
Then again, I can never resist a challenge, especially if the guy at the LBS says it can't be done.
There is the new index compatable cable housings to think about. The old stuff has too much compression to it. It's also no good as brake cable housing so you have to get both kinds.
Get the shifters and rear derailler and try it out. If you hate it, you can go back and sell the new bits. If I remember right, the space between the clicks is the same on the 6s as it is on the 7s so you can use a 7 speed shifter and just not use the last click.
Also, look at the space between the smallest cog and the chainstay, seatstay and dropout. Is there enough room in there for another cog (remember the chain will overhang this by a mm or two) Sometimes a 7 speed upgrade is really simple.