I was in a situation very much like you and your ex-leasers. I had lock ons on some private land and I received a call that they were going to be logging the property during the spring/summer months. Since I knew the loggers would not care for a stand in a tree, I got my butt up there and pulled them all out just incase. If I would have left the stands in the trees and the loggers proceeded in cutting the trees down with my stands in them, It would have been my fault and not the loggers fault.
In your case when the leasers were told about the changing of the lease and that the land may be sold or logged, they should have removed their equipment ASAP.
I would give them their property back but I surely wouldn't take any of their $hit at the same time.