You should be able to use your existing SD card in your new Kindle without any problems.
For "download limits", I don't know what the exact error message is that you saw. But it was probably a maximum number of devices that Amazon will let you download your books to. I'm not aware of any limit to how many times you can re-download one of your books, just the number of devices.
So when you get your new device, go to your Amazon account and
deregister your old device which will free up a spot for your new one.
