I'm in an old house with a chipboard kitchen and no money to spend on a new one. While i've replaced most of the crumbly cupboards lower down, we have this custom wall cupboard which is fine for now - it just needs to be a different shape.
I want to cut out the chipboard about where you see the plates sitting so that this cupboard is tall, and then put a side in and a door on (clearly there's already been a door at some point in the past). I'll then seal and paint the interior, maybe add some lights in the top, and do something about that horrible bumpy liner stuff (that's a future me problem).

This shelf is already above head height, and its in a low ceiling area (2100) so it's an awkward area to work in - I'll likely be cutting from underneath. I'd like the cut to be as straight as I can get it so that when I add a dividing wall to the right I can reattach the remaining shelf to it to keep it solid. What tools or techniques would be best?