How can I transfer data from my new iPad to my old one?

You can only transfer or restore data to a target device that has the same or later version of iPadOS as the source. As such, if your new (to be returned) iPad has a more recent version of iPadOS than your old iPad, you'll not be able to transfer data from new to old.

By contrast, if your old and new iPad have the same version of iPadOS installed, you should have no issues.

These support pages outline the available methods to transfer your data between devices:

Set up your iPhone, iPad or iPod touch – Apple Support

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support

QuickStart will not be available if either device has iOS 12.4 or earlier installed.

“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the target device. Nothing is moved or removed from the source device.

An alternative to QuickStart is to restore either an iCloud or iTunes backup to the target device - noting that the target must have the same or more recent version of iPadOS installed:

How to back up your iPhone, iPad and iPod touch – Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Apps are not directly transferred between devices. For Apps, the transfer process creates a record of the Apps that are installed on the donor iPad - and it is this record of installed Apps that is transferred to the target (new) device. For Apps that are compatible with the target device, the corresponding Apps are automatically downloaded and installed from the App Store.

If your older iPad Pro uses an operating system no older than your new one, and your older iPad has at least as much storage, and your older iPad has been thoroughly cleansed as follows:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Then:

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

Of course, the “new” iPad is the one you’re migrating to - your older iPad.