Do I have to remove the existing 6' fence posts to install an 8' metal fence?
In terms of structure, sure.  However it will be ugly, unless you are a very good welder, and take a lot of care dressing it after.  

Remember too that most fence pipe is galvanized.  The fumes given off when you heat it are quite poisonous.

It may be a better idea to just buy chunks of pipe with an inside diameter that can slide over the existing post, or alternately, to make extension posts of the same diameter with a stub that will fit inside the existing post.  This latter method would allow you to do any welding in a shop setting (much easier).  

Or instead of welding you could use epoxy glue, or hole and bolt to join the pipes.