I thought your checking on the working plant, isn't it?

If the installed spring can has different sizes, than the spring rate or the set load could be differ. Check out the spring vendor catalogue or the detail drawing if you can.

Before any advice your second question,you must check why and how you change the spring load. The spring hanger can be variable type or constant type. Depend on the type,different approch shall be taken. Also, you need to check the restraint summary thoroughly. Check the displacement at the sustained case(too much lift-up or sagging), or the changing operating load caused lines to bend or reaction force from other support.

Spring hanger's operating load can be adjusted for a small amount of load, but it has its limitation(See MSS SP-58). So, your first approach may not realistic.