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I generally recommend everyone has a full overhaul once a year - if you ride a ton then also do a lowers service at 6 months, and that covers 99% of people. You can't tell when a damper NEEDS a service, only when it should have been serviced about 6 months ago. And catastrophic failures always come at the worst time - right before or during a big ride/race/holiday Damper oil breaks down and starts to wear out parts like the inside of the damper tube - and usually feel fine right up until the point they have done major...

Assuming your Fox 40 is a 27.5" model, the 180 mm 29" Fox 38 actually has a slightly greater axle-crown distance, so you won't have to worry about the front ride height being lower. Edit: I guess Fox doesn't like direct linking so the images come out unreadable. The relevant pages on Fox's website: https://tech.ridefox.com/bike/spec-sheets/806/fork--2017-2020-40mm-user… https://tech.ridefox.com/bike/spec-sheets/1089/2021-2026-38mm-user-spec… For the 180 mm 29" 38 it's 594 mm and for the 27.5" 40 it's 586 mm (so in fact closer to the 170 mm 38)