Land leases- able to re re-negotiated during lease period?

Say a property has 6 years left on the lease, is it possible to re-extend the lease period (say to a full 60 years), while there is still time to run, or do you have to wait til the lease is fully expired, and then 're-set' the clock? And how is this process done?

Thanks, Craig

I would recomend you to deal with the land owner and make a contract before completing your lease purchase with the current lease holder. Otherwise the owner may charge a very high price when you need to renegotiate. Especially if there is a company on the property.
Remember also that in the lease, you may have to pay a percentage of your income annualy to the landowner. Failure to pay out according to contract and any fiddling with your books may cause you to completly loose the lease.