Road Cycling - 6 speed freewheel conversion to 7 or 8 speed casstte?

Is it possible to convert a 12 speed road bike ( 6 speed freewheel) to a 7 or 8 speed cassette? I think the frame spacing is 126mm.

Can I keep the same rear derailler with the 7 speed? with the 8 speed?

Can I keep the same shifters?

Frame type is important, steel and Ti frames are springable, Al and carbon not
so much. It is unlikely to be Ti or CF in 6speed though. If Al, it is generally
recommended you not spread the frame (if you even can). Derailleur may
work, just have to try it and see if it has the throw. Suspect most will. You will
need a new hub, or wheel of course. Shifters, if friction will probably work also,
certainly 7 speed, not sure about 8spd. Steve

I've read the same thing on Sheldon Brown's site, but have a question about "cold-setting" (bending) the frame to fit a wider hub. I too have a 126mm 6-speed freewheel set up, and want to do the same conversion. My only problem is that I have an AL/steel frame. The main triangle is AL, the seatstays, chainstays, and fork are made of CroMo. Is it a bad idea to try cold setting this kind of frame? (my worry is the point where the cromo attaches to the aluminum breaking)

I've read the same thing on Sheldon Brown's site, but have a question about "cold-setting" (bending) the frame to fit a wider hub. I too have a 126mm 6-speed freewheel set up, and want to do the same conversion. My only problem is that I have an AL/steel frame. The main triangle is AL, the seatstays, chainstays, and fork are made of CroMo. Is it a bad idea to try cold setting this kind of frame? (my worry is the point where the cromo attaches to the aluminum breaking)
If the BB shell or lug is steel,it's no problem.

Yes, it is possible.

7-speed is the same hub width as 6-speed - 126mm. So you can change to this without spreading the rear triangle of the frame. 8/9/10-speed is 130mm. If you go this route, might as well go all the way to 9-speed. Another option is to get a 7-speed Hyperglide cassette hub wheel and put on 8 cogs of a 9-speed cassette. The hub is 126mm so you still don't have to spread your frame, but you get 8-speed. You will need to get 9-speed shifters if you do this.

If your rear derailer is Shimano and your 6-speed shifting is indexed, you can keep the same rear derailer. It will work in a 7, 8, 9, or 10-speed setup. HOWEVER, there are some restrictions if your rear derailer is Dura-ace - you will need to get Dura-ace levers if you go to 7 or 8-speed and you won't be able to go to 9-speed.

If your rear derailer is Suntour, you have to get a lot more new stuff to get more speeds. Shimano levers don't index well with Suntour derailers.

New shifters will be required.

7-speed is the same hub width as 6-speed - 126mm. There was ultra narrow 120 6 speed 120 hub spacing.