What would happen if you drained and filled the transmission fluid late and not at the proper interval? Should you wait for the next interval if you were late?
It depends on how far beyond the interval you are and what you're driving. If you're a few thousand miles past the change in a Ford or GM, you should be fine to change it now. If you're pushing 2x the change interval or more, it's risky with a domestic. Most would advise against it. A Toyota would probably still be fine. You don't skip an oil change if you missed the interval, you change it immediately. Transmissions are a little more complicated, but along the same lines.
Nothing good.
On American cars, going over by more than about 20k miles is too long. If you change out the fluid, you likely will loosen up all the clutch material that has broken up into the transmission and start causing problems.
Japanese brands do not shed as much material because they are higher quality. You still want to maintain your car at the proper intervals, but you can sometimes get away with later changes if absolutely necessary.
The best advice is to change the fluid on time, regardless of what vehicle you have.
I drained the transmission fluid out of the pan in my 02 Mustang GT at 69,000 miles (first time serviced). So far, no change in transmission performance. There wasn't any noticeable metal filings on the magnet also. I drained the transmission fluid out of the pan in my 14 Xterra at 47,000 (first time serviced) and there's no change in it's performance. I didn't see any metal filings on that magnet also. I haven't done a complete flush on either vehicle.
