I’m afraid that in practice, the cabins on the old stock were not adapted depending on demand. Typically, an Inverness/Edinburgh/Glasgow portion would have three coaches in which all rooms were made up as first class solo rooms with the upper bunk folded away (J/K/L), and three coaches in which all rooms were made up as standard class twin rooms with both bunks folded down (M/N/P). The top bunks could only be raised and lowered in depots, as the mattresses had to be stored safely (there was nowhere to do this on the train, and the bunk could not be folded up if the mattress was still there). So in practice, in First Group and early Serco days, with the Mk3s, once all of M, N and P were full, there was no more standard class accommodation (even if first class cabins were still left in J, K and L); it was either first class or nothing. Later (Jan 2018 onwards) each of the Mk3 coaches was arranged as a composite, with six first class solo and six standard twin cabins (with 1-6 permanently arranged as first class, and 7-12 permanently arranged as standard); the overall ratio of first class to standard class cabins remained the same.
The problem with the Mk5s is that there are only four classic/standard cabins in each coach (7-10) as opposed to six on the old stock (7-12). This is because of the extra space taken up by the first/club en-suites. This means that the capacity in standard/classic in each portion has been reduced from 36 cabins to 24 cabins; the fact that the cabins can no longer be shared with a stranger has reduced capacity even further, because once one classic cabin is booked for a solo traveller, it cannot be used by a second passenger. When sharing with a stranger was permissible on the Mk3s, there were potentially 72 standard class beds available in an Inverness/Glasgow/Edinburgh portion. Now there are potentially just 48 beds, but around 10-12 of these will remain empty in any journey because the cabins can no longer be shared with a stranger. So yes, classic will book up very, very fast on all trains (not helped by the fact that the First Class fares, even at the lowest advance tiers, are very expensive). Also, the first class cabins are not interconnecting because of the space needed for the en-suites, so family groups will invariably prefer the classic cabins.
When I was booking my ticket for the northbound Highlander on 5th November, I did in fact notice that first was £13 cheaper than classic/standard. This can happen when there is very high demand for standard but less demand for first, so is worth bearing in mind.
Despite the negative press, the service, as far as I can see, seems to be getting busier all the time and a lot more difficult to book at short notice, even on weekdays in autumn... perhaps this is the Mk5 ‘novelty factor’ at the moment but I will be interested to see what loadings are like in Jan/Feb/Mar (the quiet winter months when the novelty has worn off), and whether it gets any easier to book a berth at short notice then.