So my question to you is, how is telling the OP to change one string at a time helping? She was trying the flats on another bass not changing strings. So how exactly would she go about doing that one string at a time?
I'm not saying its a bad thing to change strings one string at a time, I simply said its unnecessary. I have never put a fresh set of strings on a guitar or bass, changing strings one string at a time or otherwise, and not gotten some minor fret buzz. Either way you are changing the tension applied to the neck. The myth is that you can damage the neck by removing the strings completely, which is simply not true. Just like the myth that you should only adjust your truss rod 1/4 turn and give it a day to settle before adjusting more.
I was not responding to the OP, I think I made that pretty clear. I was responding to those who were jumping on people saying that it is a good thing to change strings one at a time.
If you read back the responses, people said that her issue would probably have been minimized by changing strings one at a time. This could have easily been achieved by doing a one string at a time string swap between the two basses, assuming both basses had strings. Even if the strings were very dissimilar, you would still be minimizing the changes in forces being applied on the neck and this minimizing the setup work after the fact.
In the end, even if the strings are dissimilar, there is still an advantage to doing a one-at-a-time string swap because you are still minimizing one variable in the change you are applying on your instrument.
I don't think people were out of line in expressing their opinions on what may have exacerbated the OP's situation.
By the way, the 1/4 turn thing is not a really myth or an absolute rule. It's a great precaution because when doing it, you have no real way to of knowing what condition the neck and rod are in, even if you built the bass yourself. Wood is not perfect, rods are not perfect, no matter what a manufacturer might lead you to believe. Do a 1/4 adjustment, then wait for it to settle, then reassess the situation. You could probably do more than a 1/4 adjustment nine times out of ten, but why risk a perfectly good neck on the assumption that nothing will go wrong.