I would like to replace a 6 foot fence with a 8 foot fence. The current fence has metal post. Will I have to dig up the current post or can I weld an extension to the existing post.
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.