It is very unlikely that you will be able to trade-in your iPad Pro for a different model without incurring considerable cost. Potential trade-in values are much lower than the retail cost of a refurbished iPad. You might consider selling your unwanted iPad privately - as this may realise a considerably higher value than a trade-in with Apple.
You can verify eligibility for trade-in and obtain an estimate of the potential trade-in value here:
Apple Trade In - Apple
You can also trade-in at a physical Apple Store - many choosing this option by preference as the device can be checked, the trade-in value finalised and the process completed while in-store. Be aware that Apple’s in-store trade-in values may differ slightly from the on-line estimate.