I replaced mine a yr or so ago, so I've forgotten the details, but I recall a process for initiating the new thermostat. I followed a diy on "disco3" forum. It was something like turn it on and rev RPMs to 2000 for a minute. Turn off and bleed the coolant. Turn on again for some specific time and rpm. Bleed again. Etc.
It seemed kind of silly and I was wondering if somebody was doing a spoof, but it wouldn't do any harm. Anyhow, even after doing the process, I got a CEL light once or twice in the days after, but I kept a close eye on the temp and checked to ensure that coolant was circulating--any mistake here could end up costing a new engine-- and reset the codes and it's been fine for many months since.
Not the peace of mind you were hoping for before a trip, but it may be false alarm.