If my Pokémon GO Plus + has already been paired with a smart device, can I pair it with a different smart device?

Yes, you can.
Please unpair your Pokémon GO Plus + from the original smart device, and then pair it with your new smart device.
Please see here for more details about how to pair or unpair your Pokémon GO Plus + and your smart device.

If your Pokémon GO Plus + is unpaired from your device, you won’t be able to retrieve any sleep data currently on it.