In this transient thermal model, you have a temperature-dependent thermal conductivity material and you want that material property to permanently change on an element-by-element basis after the element crosses the 150 C threshold.
I’m not an expert at APDL, but I believe code could be written that executes after each time step to change the material assignment of any element whose current temperature was above 150 C to a different material that has a constant value of thermal conductivity. MPCHG is an APDL command to change material. You need code to select the elements to apply that change to. A simple loop through every element with a simple IF statement that checks if the temperature is > 150 then changes material would do it. Maybe there is a command to get all elements that meet that condition without needing a loop.