Hello Where in the rules does it cover subs changing depth? I assume in 20 min it can change the full depth range. Also is layer depth specified in the scenario, ie. between shallow and deep?
Yes, with a turn length of 20 minutes all submarines can achieve their full depth capability, so you can set whatever depth your sub is capable of each turn.
For the depth of the layer and how it affects sonar detection in MNC look at Rule 184.108.40.206 and Table 220.127.116.11 'Cross Layer Check'.
Note: While the Thermocline boundary layer does vary in depth to a certain degree, with the broad depth categories used in the rules the layer is assumed to always be between the 'Shallow' and 'Deep' depth ranges, as the variation of the layer is less than the difference between the two depth ratings.