This happens all the time. The water will pass through the old jet and into your new jet. I personally have one stuck in a well I maintain and it works just fine. I always leave off the footvalve and just let the internal footvalve in the jet do the work since there is already 1 regular f.v. and one internal f.v. built into the stuck jet. You'll see what I mean when you install the new jet and water comes out of the drop pipe.
Cut off a few feet and make sure you use brass. If you need any help etc. PM me, I can either find what you're looking for or send you in the right direction. You would be much better off going to 1" PVC drop line with special adapters to attach to the jet/well head.