Hello Richard,
Can you elaborate How Caesar Deals with Spring Supports during Alt Sustained Case.
Although we are not checking support loads here,but the concern is : Are the Springs getting redesigned, and leading to a different Stress values in these cases?

We generally do Stress check in Support Lift-up cases while keeping Springs User Defined in that Caesar file.

(Rich is teaching today...)
No, any spring sizing/selection precedes this Alt-SUS case. The selected springs will be included in every analysis (unless you fool with them in the load case editor, e.g., locking springs for the hydro case).
Regarding the spring preload - spring preload vector is applied with the load case primitive "H" (as in W+T1+P1+H). The full preload is applied, and rightly so, in any load case including H. With a properly-adjusted spring hanger, that initial load will always be on the system; the total spring load will be composed of this preload and the change in load. The change in load = the vertical change in position times the spring rate.