can I cut in / blend old brick extension
Yes, you can cut out and bond in new bricks to get a seamless match in the bond (assuming the gap is not an expansion joint - if it's mortar-pointed it's unlikely to be; an expansion joint should have sealant in it).

The main issue that you will run into is likely to be the ties used to fix the new wall to the old. If wall starters were used, they'll run right the way up the height of the wall in the joint and be in the way of where you are looking to put the new bricks. As you are bonding it with new brick, you won't need to worry about replacing any ties you have to take out.

Re-facing the bricks will be a hell of a job - the bricks will be hard and it won't be easy to tacke the face off in situ. Plus you'll need a diamond saw to cut faces off the new bricks (or to buy slip bricks from a merchant - most will be able to get these to order). My biggest concern with that would be ensuring that they are suitably bonded so that the faces don't blow off with freeze-thaw.