Dear JLC,
When your predecessor in title signed the lease, you are bound by the terms of the lease. Here is an excerpt from the most prolific lease form in the State of Texas, which is likely similar or exactly what is on the lease form that was executed by your devisor:
"7. The rights of either party hereunder may be assigned in whole or in part, and the provisions hereof shall extend to their heirs, successors and assigns; but no change or division in ownership of the land, or royalties, however accomplished, shall operate to enlarge the obligations or diminish the rights of Lessee; ....."
If you have not read this blog post, it might be of some help to you:
http://www.mineralrightsforum.com/profiles/blogs/i-inherited-some-producing-minerals-what-next
Best wishes,
Buddy Cotten