Easy way to cover old skirting boards with new ones?

Hi There,
I'm renovating a place with Brick walls, where someone has covered the brick walls with plasterboard using masonry adhesive, but sat the plasterboard on top of the old skirting boards.
The old skirting boards protrude from under the plasterboard at varying depths along the walls, in some places up to 1/2 inch.
I think trying to remove the old skirting boards would damage the plasterboard far too much, so I want to put new (taller) skirting boards in to cover the old ones, and somehow rebate the back edge of the new skirting boards so they cover the old skirting boards and fit flush with the wall.
What's the best way to carve a half inch x 2 inch rebate in the back of the new MDF skirting boards? Electric planers seem to only have a cutting depth of 3mm, and a router would make a lot of mess - I have to reskirt 4 medium sized rooms!
Any ideas on the easiest way to do this?
Thanks,
Mark
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Consider setting the base board, then adding a trim piece on top to bridge the gap. In fact, you might use a rotozip or similar reciprocating saw to cut alway the botttom of the wall board and slide a quarter inch piece on top of the existing base board. It would create a nice shadow line.
We have this sort of arrangement in our 1834 house and it looks good.
TB
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