It is worth to explain one of the the main reasons (and not the only one) why it is best not to untune all strings of the guitar at once but if possible change them one at a time. Many guitars (as the Spanish traditional method of construction has it) have their frets being engraved into their narrower sawn slot by tapping with a hammer and as the base of the fret sits into the narrower slot it provides a constant pressure to the fretboard at that point on each adjacent side of the fret
<cut explanation> (I've read it
5th to 2nd and 4th to 3d, so that evenly and in a slow rythm the tensions are subtracted from the instrument, clean the fretboard immediately if possible and replace and tune immediately if possible the strings. The shorter the time period the guitar is left completely untuned the better.
When you build a guitar, and I've seen you build fabulous guitars, can you notice the neck bending backwards after fixating the frets in their slots?
I did notice the bending of the neck somewhat when stringing the guitar, but when removing the strings, doesn't the neck just bend back to its natural position?
When I change the strings, I unwind them all untill there is no tension left on the strings. Then I just cut the strings off at the bridge and at the rollers, so I don't have to pull the complete string through the holes at the bridge.
Then it's time to clean the fretboard and the soundboard at the bridge.
After putting new strings back on (the old ones are a bit short now
), I bring them back under tension smoothly and evenly.
I never do this
I've the experience that because of the thickness of the thicker strings the smaller strings can come loose or are not so fixed as can be.