Most well made guitars are set up and adjusted with all the strings up to tension. As the strings are stretched, the forces are distributed and stored in the guitar’s body. The top, sides, back, and neck move together in order to absorb this energy. With a minimal amount of built-in compensation by the maker, the guitar reaches a perfect setup once the strings are at pitch. The question at hand:
Is it safe for your guitar to have all the tensioned strings removed together or is it better to only change one string at a time?
When all the strings tension is released the guitar relaxes. It’s common thought that quick changes to an instrument are bad. The removal of the strings, however, is a change that your guitar is used to. When your guitar was assembled and the last few steps were made to close the soundbox, it was done without being under the tension of the strings. As a result, the guitar goes to a state that is familiar to it. The arch of the top settles down, the neck relaxes and flattens out, and all the stress that was distributed to the back and sides is lost.
It may not be a bad thing for your guitar, but it may be a problem for you. With the strings off and the guitar body in a relaxed state, getting it back to its normal, equalized state will take a bit longer than it would when replacing one string at a time. The wood in your guitar is most happy to get back to the physical setting that it has spent most of its life. If you plan on performing I wouldn’t suggest changing all the strings, unless you have a window of at least a couple days to work with them. I wouldn’t suggest changing your strings right before a performance anyway.
Your guitar needs to be cleaned and conditioned at least once every year
Conditioning your fingerboard can prevent cracks and unwanted movement in the ebony or rosewood as well as maintaining a nice surface for your fingers. Also, polishing your frets and removing oxidation can make your guitar more enjoyable to play. This type of work is best done with all the strings removed.
The guitar is designed to have strings on it with sufficient tension. When working on guitars it’s necessary to take the strings off to make things both easier and faster. With other guitars, such as archtop guitars and electric guitars with floating tremelos, the worst thing about removing all the strings is that you loose the guitars setup.
This is usually necessary in my line of work, but it can be the worst nightmare for many players. These systems are the focus for future writing but it simply takes more time and effort to get them back to the perfect setup than it would be to maintain the setup and replace one string at a time.