I just changed to 10 speed so I could get a clutch rear mech - keeping my 9 speed xt crankset. I changed the chain rings as they were quite worn. The problem is I wanted 22, 34, bash. But Shimano dont do these options in 10 speed - the smallest ring you can get is 26 - bit of an oversight I feel, certainly no good for the Malvern Hills where I live. I thought about buying new 9 speed shimano chainrings but they are not recommended for 10 speed drives.
So in the end I bought Blackspire rings as they come in the right sizes and are rated for 9 or 10 speed. They are also quite cheap on CRC
I may be being a little naive here - perhaps Shimano are more diligent with their tolerances whereas Blackspire just decided to add "and 10sp" to the description of their old rings. After all - how can one ring be ideal for 2 different drives? (One ring to rule them all.....) so you would probably be OK with Shimano 9sp – but not optimal. But Blackspire do come in pretty colours