Is there a way to plug up the gas port at one position and then redrill at a new position?  If so, what are the details?

Not at home and probably not even at a good gun smith can it be done safely.  I do think it would be nice to move the gas port up another 4 inches in a 24 inch barrel though and put on one of those really long free float tubes to have an extended sight radius thats more durable than all those bolt on front sights

I have to agree with Jonathan.  Unless you have good tools and the talent to use them, you run the risk of just plain screwing it up (canted gas block, etc.) or ending up with a questionable/dangerous setup.  I'd sell the old barrel and just buy what you want.

Having no model to work from and pulling these thought straight out of my A--, here's how I would do it if I were to try something like this.  I'm going to assume that you want to move a carbine length gas system out to either midlength or rifle length.  The numbers I am using are from memory and may not be accurate.  YMMV, FWIW and any other disclaimers also apply.

The standard gas block profile is 0.750" and the barrel forward of the FSB is USUALLY something less than that, so that rules out using any standard sized FSBs/gasblocks on any part of the barrel forward of the standard gas block area.  This leaves you with reduced diameter gas blocks, of which there are standard 0.625" FSBs that come on pencil barrels and PRI makes a 0.625" version of their flip front sight.  I would drill and tap the old gas port for a very small setscrew, install the setscrew and weld over it.

You would then need to determine where the new gas hole would be - midlength or rifle.  Then, using a lathe, you need to profile the barrel from the muzzle to your intended gas block area to 0.625".  Then you need to drill your new gas port in the appropriate spot, to the appropriate size.  While profiling, remember to leave a step for your handguard cap, if you are using one.  Anything significantly over 0.625" will do to keep the handguard cap in place.  While you are profiling the barrel, you might want to clean up the weld on the old gas port.  You will end up with a barrel the has the pencil profile from your new gas block area all the way to the muzzle.  If your barrel muzzle is threaded, that will save you some work also.

Then there the project of installing your new FSB.  PRI is the easiest, but if you want a standard FSB with taper pins, you have alot more work to do.

Another thought crossed my mind in that you MAY be able to squeeze a 0.750" PRI flip sight gas block enough to seal a barrel where the profile is a little less than 0.750", but I wouldn't count on it.  Let's say that the barrel profile forward of the FSB is 0.740" and has the typical phonographic finish that many barrels have.  You would have to turn that smooth, using a lathe, to maximize the sealing area of the gas block.  Let's say you loose 0.020" in the process, so your profile is down to 0.720".  The PRIs I use are almost squeezed to their maximum when installed on a standard 0.750" gas block profile, that is why I have my doubts that they will squeeze down much further.

I'm sorry it is so long winded, but I just wanted to let all know what they may be up against should they try something like this.  Plus there is no guarantee that the damn thing will even function properly when your are done.  It as likely to KABOOM as it is to function properly when done.

Buying a barrel is much easier, but if you are a tinkerer, like me, projects like this sound interesting and challenging.  If I had time, I'd try one like this, but time, and money for that matter, seem to be in short supply these days.

Good luck!