Yeah it's always a concern that the school kids @Hellfrauds won't know what they're doing
i have done it a few times before on my previous car's but they was before ABS
No issue with ABS as long as you don't allow air into the ABS pump or drain the system. If I need to disconnect a pipe or change a hose, I always remove the reservoir cap, cover the reservoir with clingfilm and replace the cap to create a vacuum and inhibit fluid loss. If changing a hose, clamp both the old and new one, and have everything to hand to change them over quickly for the same reason.
Renewing the fluid is a quick and easy job with a pressure bleeder. You're just pushing new fluid through until you can see it at each bleed valve, and no air should enter the system at all. I must admit that I always slacken the wheel valves very slightly - just enough to break the torque -
at each service (once a year) and retighten them even if I'm not changing the fluid, to minimise the risk of seizure. I always remove any surface rust from the caliper around the bleed screw first with a wire brush. Always use a proper brake spanner or socket with six flats rather than open ended or multi-point spanners to minimise the risk of rounding off the flats. If necessary, I have found Innotec Deblock Oil to be the best deruster and penetrating oil and far superior to WD40.
It's not been my experience that Volvo dealers don't change brake fluid. Admittedly my car only went in for dealer servicing twice from new, for the warranty cover, but they weren't shy in trying to upsell brake fluid changes as a quick way of making an extra £50 or so from the customer at any opportunity, for a few minutes extra work whilst the car was already on the lift. Same experience with other makes that we have used.
It's notable that even the manufacturers and suppliers of professional brake bleeding equipment use the additional revenue stream "opportunity" as a selling point to garages and workshops.