7 speed gearing system changed to higher?

[Matthew_T]

I currently have a 7 speed rear mech with a 7 speed cassette.
My question is this: If I were to change it to a 8/9 speed (more common), would I have to change the downtube shifters as well? When I change gear, I can hear the shifters clicking, not the rear mech.
What else would I have to change? Front mech, chain, rear wheel, whole frame?

This is just a query as I am not going to change it unless I come across a big stash of cash or the current system fails completely.

 

[RecordAceFromNew]

It is impossible to give you a short yet reliable answer without more information.
1) Are you sure you have a 7 speed cassette rather than freewheel?
2) Is the rear hub's over-locknut-distance 126mm or 130mm/135mm?
3) If 126mm is the frame alloy or steel (use a magnet)? If steel do you know what tubing?
4) Regarding the right/rear downtube shifter, is there a knob to change from indexed to friction? If you don't know a close-up photo will help.