My XC60 came with 20" wheels but I took advantage of the Volvo deal for a set of 18" rims with winter tyres (Nokian WR SUV4) for £1366 all in.
Fitted them last weekend and after calibrating TPMS and ~700 miles during the week I can honestly say other than being a slightly smoother ride (if that's possible on the air suspension) there's no noticeable difference between the 18s and 20s. Don't see any need for a s/w update. Plug the wheel/tyre dimensions into this site:
https://www.tyresizecalculator.com/t...ire-dimensions
In my case, the key point seems to be that the 20" wheels will rotate 432 times/km whereas the 18" will rotate 431 times/km. I'd call that pretty insignificant and not worth the expense of a dealer visit.
Steve