Your ground lease should allow for the lease to be assigned to others without permission, just as the land owner can assign their interests, as well. It has been my experience that most land owners like you to sell to a bigger company, as it makes their money seem more secure. Who wouldn't rather have their rent coming from a huge company like Lamar as opposed to a smalll start-up, right?
Another reason that a land owner should be happy with you selling your ground lease to a bigger company is that most leases offer a percentage of gross rent to the land owner, and the bigger company can rent their ad space for more than you can. Considerably more. A national advertiser might pay two or three times more than you're getting from a local advertiser, in some cases.