If I should not de-tune guitar for shiping, should I then change strings one by another, i.e. take E down and put new E in, then D etc? Frankly, I have never done like that, only all 4 strings down and then all 4 new in.
That is the way I do it, one string at a time. the only time I take all the strings off is when I need to for changing pickups, nuts, bridge work, etc. When you take all the tension off, then bring it back up to full tension the neck may not come back to the exact same place, and could potentially need a truss rod adjustment. It may work fine most of the time, but the potential for problems is there. It is just safer to change them one at a time whenever possible.

