Can I replace post mount disc brakes calipers with flat mount calipers?
I think there are no adapters on the market to use flat-mount calipers on a post mount fork, so your options are:


Get a flat mount fork and Ultegra caliper you want. I actually did that, the additional price will depend on new/secondhand and no-name/open mold/brand name factors.
Get any post mount Shimano hydraulic caliper, those are all compatible. There's no actual Ultegra post mount calipers, but prior to R8000, Shimano made a non-group, Ultegra-level ST-RS685/BR-RS785 shifters/calipers, so you might use a BR-RS785 or slightly older BR-R785 (actually an XT).