What would be likely to go wrong (initially or over time) if 50mm concrete block paving were laid on a *thin* (perhaps 20mm) bed of sand over concrete? Or on a thin mortar bed over concrete? Only traffic will be very light pedestrian - it's a short path in a back garden.
I ask as the path has grotty 450mm slabs laid on top of what looks like a pretty solid, if old and rough, concrete base. Removing slabs and mortar would be easy . Removing concerete and laying a conventional base for blocks would be a pain. Even just sticking a thick layer of sand on top of the base would I think leave the top too high.