9.5 ah replaced with a 8ah battery?
Amp-hours is a measure of the battery's capacity, and more is better. The 9Ah battery can supply, for example, 1A for 9 hours while the 8.5Ah one can only supply 1A for 8.5 hours. The important thing is to match the voltage of the battery, which in this case is the same at 12V.

So there's no reason not to get the 9Ah battery -- it's the better one.