Condition 15 of the National Rail Conditions of Travel disagrees with your assertion.
No mention is made of a requirement to indicate the presence of First Class in the eNRT.
It doesn't need to be; it's simple contract / consumer law. This is why the longstanding principle exists, that if a train is advertised as only conveying Standard accommodation, then any 1st class carriages are contractually considered to be Standard class.
If they are not locked out of use, then they can be used by any passenger holding a standard ticket.
I don't see why Chiltern need to make this difficult and act in a combative manner during this transition period; all they need to do is advertise the relevant trains as conveying first class and not price any first class fares on the Chiltern route, therefore only charging supplements on board when it's available. On rare occasions, a passenger holding a longer distance 1st class ticket may use such a train and be entitled to some compensation, but that would be miniscule.
Alternatively, simply declassify them, as already done.
Chiltern used to get things right, so there is no need for them to change and start becoming combative. There is just no need for it.
If they do start denying carriage to people holding Standard class tickets, and they do this to the wrong people, they could end up with an expensive court case. Some TOCs have had to pay out thousands of pounds in compensation when denying people their contractual rights over various matters recently (e.g. enforcing non-existent time restrictions), and I would advise Chiltern to exercise caution, and act in a sensible and lawful manner, if they don't want to be in that position over this matter, as I could easily see that happening.
