6-speed, 7-speed, 8-speed, 9-speed, 10-speed, 11-speed?
From this article, it would seem that the problem is in changing from the 7 to the 8, you probably would have been better off going with a 9 or even a 10.
Quoted from the above link:
Bicycle Gears: 6-speed, 7-speed, 8-speed, 9-, 10-, 11-, ...?
Component manufacturers like to sell you lots of new parts, even if you don't need them. This has led to much confusion as various parts are labeled as if they are incompatible with other parts even though they are actually usable with little or no problem. Also, design often is churned by spec hype, and "keeping up with the Joneses," as in more sprockets, lighter weight, higher-priced components must be better. "Jones" is also a slang term for a drug addiction!...
There are, on the other hand, some real compatibility issues.
The following parts only are "speed specific":
Indexed Shifters These need to have the spacing of detents ("clicks") to match the system they'll be used with. This usually goes along with the correct number of clicks -- though a shifter with an extra click also can work, as long as the spacing is OK. (Friction shifters have no compatibility issues, they work with everything.)
Cassettes It is really the cassette that determines how many speeds you have in back.
Campagnolo/Shimano 8-speed cassettes have different spacing, so you can't generally get good indexing using a Campagnolo 8-speed wheel with a Shimano shift system or vice versa.
With 7-, 9- and 10-speed systems, the sprocket spacing between brands is close enough that it rarely causes any difficulty in practice.
For perfect matching, you might substitute different spacers, use alternate cable routing, or use a Jtek ShiftMate pulley adaptor.
See my "Spacing Cribsheet " for more details on this. (go to the website for the hyperlink)
Chain As you go to more sprockets on the cassette, you need a narrower chain. However, using a chain one size narrower than standard rarely presents any problem. Thus, you can use a "9-speed" chain with a 7-speed or 8-speed system, or a "10-speed" chain with a 9-speed system. This is not the ideal approach -- shifting may not be quite as smooth -- but it's workable.
Narrow chains bring other problems, though. They are usually more expensive and -- with 10 or more speeds -- don't last as long -- even when used in the intended system.
The narrowest chains also are more trouble to maintain. A master link, the SRAM PowerLink, makes it easy to disconnect a chain for cleaning. The 7/8 speed SRAM PowerLink works with SRAM and Shimano chains, probably others too. The 9-speed PowerLink works reliably with SRAM chains, but it may lead to a Shimano chain's jumping forward. The 10-speed SRAM Powerlock (note different name) is good for one-time use only: You must install a new one every time you reconnect the chain -- but then, if the chain has worn significantly, it will cause a "clunk" every time it comes around, because this one link is shorter than the others! Shimano's 9- and 10-speed system is even more trouble: you must press in a special link pin, using a special tool, every time you reconnect the chain, and this, too will cause a "clunk" if the chain is worn.
The article goes on to discuss the derailers and other aspects of speed performance.