The problem is that the position of the Smart Connector and of the camera bump changed, due to the size increase of the iPad Pro. It likely can’t work. Apple probably also disabled the compatibility on the OS level to prevent support issues due to the misalignment.
Even if you get the connector to line up, do to completely different dimensions and likely slightly different places of where the magnets are at, it’ll probably just fall the second you let go.
Likewise, if you put an older iPad on the new keyboard, it’ll probably just topple it right over.