Just inquiring: Do you ever mix up dragon alignments in your games of 1e?
Nope. Never felt any need to. I can't even specifically recall doing that with ANY monster in D&D, though I'm sure I must have at some point. It severely undermines one of the purposes of alignment if players otherwise have every reason to expect alignment A but you instead substitute alignment B. With incredibly few exceptions, DM's who I have heard of doing so have only done it to unpreventably trick PC's into doing really bad things because of otherwise reasonable expectations, and then seriously punished them for not having detected the DM's scheme to do it.
I would ask, if a monster is going to be labeled as a given alignment, WHY would that monster be given that alignment? What is alignment for if not for
everyone to use to understand and predict behavior? Alignment exists at all so that the DM has guidelines for how that monster behaves and for the PC's (if they are given reason to do so when immediate actions or attitudes don't add up) to detect good/evil or actually use Know Alignment and THEN similarly have a guideline for how that monster can be expected to behave. If you then secretly/spontaneously change that for a monster (including by saying that their alignment itself cannot be predicted), especially KNOWN monsters which the PC's have met before and have certain understandings of how their alignments are assigned, you're just digging a pit for the PC's to fall into without a clue that the pit even might be there. For example,
it is a known thing in D&D, the "chromatic" dragons are all evil and the "metallic" colored dragons are all good. You turn that understanding against the PC's at your peril if you're the DM because they can no longer USE alignment as the RELIABLE gauge of what ANY species of monsters are going to do.
There are only a couple reasons to do it. One is the
RARE exception. You perhaps make one beholder in your entire campaign setting into a LG bartender, or a SINGLE, INDIVIDUAL silver dragon to be one that has become evil. But you have to be careful not to just toss all the other alignments out the window for beholders or dragons in general without providing proper and adequate information TO THE PLAYERS, about what changes you might make and how they might be expected to deal with the new uncertainty. If the PC's meet a dragon are they supposed to just flee both LG and CE dragons? How can they possibly know which they might be dealing with until they're all ROASTED where they stand. Before they would have at least been able to say, "well it's metallic in color - unless it's a one-off creature the world has never heard of, it's going to be good-aligned". But now they get to deal with every dragon as a complete enigma.
So then there's the second reason. It's fine to do it if you WANT that and the players are willing to accept it, simply to have a significantly altered campaign setting, but alignments of monsters are noted for good reasons. They're SUPPOSED to behave in a certain way, predictably (at least after the first-time-ever encounter with them) so that
Detect good/evil and
Know Alignment are
required in
every encounter before PC's know what to do. If you want all silver dragons to be evil and all black dragons to be good, go for it. But give players appropriate heads up about such things. Don't just spring it on them without warning. And certainly don't just make alignment a random determination without REALLY good reasons for it. Give PLAYERS the chance to understand why it might be, and the opportunity to protect their PC's from potential disaster due to simple ignorance that
you put upon them.