This question already has an answer here:

Considering the present truss rod adjustment is good (Vintage Gibson SG with very old strings), is it generally acceptable to remove all the strings before restringing? It's just so much easier to recondition the fret board this way. I'm planning to restring with medium gauge cobalt.

merged by Doktor Mayhem Aug 2 '15 at 20:48

This question was merged with Changing the strings: one by one, or by taking all strings off at one time? because it is an exact duplicate of that question.