How do I deal with a sloping roof?

My advice would be to dismnatle the affected cabinets after marking the gable and base position on the wall with a pencil and level. Mark verticals and horizontals.
Offer each gable up to the bottom edge of the slope and determine the scribing height by measuring the distance between the bottom of the cabinet and the pencil line marking it’s finished position on the wall. Ensure that the gables are aligned vertically with their final positions as indicated by the pencil lines.
Using a relatively sized scribing piece and a pencil you will have to transfer the shape for cutting to the gable. This is a two handed job and the services of another will be required to hold each gable in place whilst you transfer the shape of the slope to mark the gables.
Cut each gable with a jigsaw and you may need a little fine tuning to accommodate the contours of the plaster, until the gable is sitting flush with the final bottom position as pre-marked on the wall. Ensure vertical alignment is plumb then all you have to do is to reconstruct the cabinet suitably cutting affected parts, i.e. top board, back, to the relevant depth or height.
Hope this has been conveyed in an understandable fashion.
Do let me know how you go on with this,

Tim.