curious if you meant 7mm/9mm, or if you meant size 7/size 9?
7mm is just over size 10.5
9mm is size 13
these are farther apart in scale
size 7 is 4.5mm
size 9 is 5.5mm
these are closer in scale
if you meant size, then the mm diam isn't that far off. you could just eliminate a few stitches as salmonmac suggested (if there's a small pattern repeat, maybe just delete one of the repeats?). or if the pattern has a small/medium/large sizing, you could do medium instead of large, since you'll be using larger needles which already makes a looser/bigger fabric. always lots of wiggle room. good luck!