Transgender

You should basically never use a trans person's old name, gender, pronouns, etc. unless you absolutely have to. Treat them as if they've always been the person they are now.

There are of course some exceptions, like if they're not publicly out yet. You should never ever out someone else unless you have explicit permission. Or if there's just no other way to get the information across, e.g. if you're describing them to someone who only knows them pre-transition or if their legal name is required for something and they haven't changed it yet. If they explicitly told you they don't mind it's obviously okay too, but that's probably rare.

In short: Just keep in mind that bringing up their old identity is uncomfortable or even painful for many trans people and act accordingly.