Hmmm....
this is how i envisage it.
Either way you'll have to cut down one of the new end panels. I'd prefer to cut the cupboard end panel as in the my first suggestion (pic) so that the full height panel comes straight down from the wall edge to butt up with the new end panel (that has been cut to fit). No need for any kind of beading.
The main issue i can foresee is that you'll need to pack out the depth for the full height panel. You aint gonna be using plasterboard adhesive.
Grab adhesive might work, but you've got a heavy panel to secure.
Screws? Well they'll show and look fugly.
where i've marked 'join here' is where i'm not sure how it'll look, won't i have a piece of wood stuck to the wall with a visible join all the way up the side of the wall?
Aahhh.....ow you show us the tiles on the other wall.
The join will only show half way up the wall.....to the worktop as its only butting up to the other end panel.
The full height panel effectively forms the 'return' from where it meets the tiles.
You could always trim a bit back on the top half where it meets the tiles and put a bead from the top of the tiles to the worktop just to form a nice corner join.
Well then why not cut off the full height end panel to the same height as the tiles, then plaster the rest of the wall above it. should be much left to make good surely.
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