Feel no fret (huh!).
It's the worst of myths that still perpeturating the net. that:
a) you should not change all of the strings at once, but one at a time.
Not a total myth. Changing them one at a time makes it easier to tune up the new string by checking against the other strings.
b) you should take all off the strings when storing it for a long time.
Not a total myth. It makes sense for a classical nylon string guitar with no truss rod but not for any instrument with a truss rod. If you have a truss rod, leave the strings on and tuned to pitch. Or take the strings off and release the truss rod.