Cyclocross bike brakes - change from cable to hydraulic disc brakes?

I've been thinking of going hydro for ages on my gravel bike too - for me it's likely to be SRAM as you can still get post mount quite easily.  My biggest problem though is that i'd have to go 11 speed with the SRAM which will mean a new cassette and chain as well at the very least...money money money...

Depends on your bikes age as well. IIRC if it's post mount you'll only be able to do newer 11 speed Tiagra (or is it 105?) for post mount, and no Sram options.

Having two identical bikes except for one being fully cable TRP Sprye and the other hybrid TRP HyRd - the hybrid is a big step up in stopping power despite still using steel cable (and despite using the same pads and discs). However, it is a bit more on or off, ie. less modulation. Suspect full-hydraulic gives you the full set of power and modulation.

You really need to give some idea of what you're trying to fix by thinkig of going to hydro. 

Giant do an upgrade kit for around £120 - replaces your stem with the hydro resorvoir and includes all hoses and calipers. Potentially an absolute steal at that price

If not take a look at the Yokozuna/Juin Tech R1 hybrids recently reviewed here. Having just gone from mechanical to these the difference is very noticeable and a worthy upgrade. Big advantage is that they're a direct swap for your mechanicals, will probably take no more than 15 minutes to fit and adjust. Similar price to the Giant system

To go full hydro you're looking at £300 and a lot of work as you need to replace the levers.

You'll need levers cables and calipers to do it properly. I've got one bike on cable brakes and one one Hydro. Cable work fine when set up properly (take it to the LBS and save yourself an afternoon of twiddling) but I much prefer hydraulic.

Hybrid systems make no sense whatsoever. They cannot physically offer better feel, power and modulation because still rely on a steel cable... The only single advantage is self adjustment for pad wear.

Correctly designed, executed and set up mechanical system can be at least as good as hydraulics. Personally I prefer my Paul Klamper with Campy levers to a typical drop bar hydraulic system e.g. from Shimano.

As Halfwheeler hints, a solution which will be much cheaper and easier would be to fit TRP's HyRd brakes, which are cable operated as far as the calliper but hydraulic thereafter.  They won't be as utterly brilliant as  full hydraulic but I've heard people say they are much better than cable-only discs.

You would be able to keep your existing levers, so saving the major expense of swapping to full hydro.

I would also say don't listen to anyone telling you that such-and-such cable brakes are 90% as good as hydraulic.  No they are not.  They are just not.  Nowhere near.  A mile off. 

I know there are some hydraulics that are cable activated, kinda neither fish nor fowl, but most hydraulic brakes have no cables. The brakes are activated by fluid running from a reservoir on the brake lever to the calipers. So you'll need the whole shebang; levers, hose and calipers.