Change from shallow to marcellus

We have a lease with less than a year left on it.  The original lease was for several shallow wells and we are receiving a very low rate per acre.  They came in, did alot of prep work but have not drilled.  Now they are telling us that they are going to drill a marcellus well.  My question is since they are the ones that changed their intentions with the lease do we have the right to renegotiate for a higher rate?  They have told us if they do not drill before the lease is up the will give us a better offer on the next lease.  The way I look at this is the lease is all we have for sure at this point and that is what we need to address.