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If you find wheels that are 9.5 and have an offset that will work you can try. The tires will be stretched and some tire shops might not want to mount the tires on the wider wheels. I have a friend that did this with tires he had owned for many years, (run flats) and he constantly ended up with flat tires because the tires had taken a set to the old (narrower) wheel width and didn't like being stretched to the wider wheel size. New tires that fit the new wheel width and he has never had a problem since.
100%, don't do it!! Move up to a larger (recommended) tire size for the new wheel width.
What tires are you running currently? and what is the actual overall width mounted?
Some 245mm width tires have 9.5" rim listed in the ranges their tires will fit, the stock GY F1 runflat has a 9.5 listed for that tire, and the actual overall width on the 8.5 is 10" wide
You will probably have a slanted sidewall with that width tire on a 9.5 rim.
Or you could always send the rims out for narrowing, that is what I did to get what I wanted, check out these guys, did the work for my Chevelle and are currently doing my CCW's for my Corvette.