Hello,
I have tried to upgrade Christminster area from absolute block to track circuit block in this save game: http://list.extended.simutrans.org/~vladki/debug-saves/demo-7.sve. The rest of the main line is absolute block. Trains do not switch working method until they stop and wait for clearance. So if the track is clear, they happily pass several track circuit signals, but in their details they say absolute block, or vice versa. Reversal, stopping in station or waiting for clearance, makes them switch working methods.
I would expect, that working method should be switched on approach to a signal (probably even a distant signal).
Thank you for the report. Does this appear to affect actual behaviour in any way, or only the display of the working method, so far as you can make out?
The reservation behavior of absolute block and track circuit is quite similar so I'm not sure if I can really distinguish between the two. But it seems that the signals behave as they should. At least it does not affect the gameplay. It might be more challenging to see how trains switch between absolute block and time interval.
Given my limited time, I will treat this as a low priority issue for the time being, but thank you for the report nonetheless.