I'd take the beads off and replace them. The ones you have on are beginning to rust, so well worth doing. You can either use 2 quality galvanised ones (Expamet), or you can go for stainless steel, more expensive, but it's up to you.
If it was me going to plaster that job, i'd put the new beads on, plumb them up, but bring them forward slightly, 5, 6mm or so, just to give you a slightly thicker coat of plaster overall. Double check the measurement "across the 2 beads, making sure it hascthe same measurement across, all the way up,, vital for the central line. I would then make a pointed template of the shape out of a thin 5mm ply, plastic or similar. I'd then prep the strip of wall to be plastered. I'd put pieces of fibre tape "across" the two vertical joints, all the way up, so you're more or less covering the whole area with fibre tape, from top to bottom, "across the way", very strong, much stronger than putting a couple of strips up and down the way.
Once the template shape has been formed, hold it against the strip of wall, using the corner beads as runners, and slide it up the wall to make sure you have plenty of depth for the bonding plaster. Once your'e satisfied you have enough depth, put a small panel pin either side of the template to keep it steady. The new corner beads will act as "tracks" for the 2 panel pins to follow on the outside edge of the beads.. Give the area you prepped/tapedearlier a damp down/pva, then start building out the plaster. Once each coat has been put on, run your template carefuly up the wall each time, removing any excess as you go, and filling out any hollows. Once you have the basic shape, leave it to pick up, then fine tune it as it begins to set. Once it has set, you'll have the shape you want, then it's just a matter of putting on the finishing using a small trowel, making sure you keep the centre line looking plumb. Hard to put into words, but i'm sure you or the plasterer will know what i'm on about.
