Flagstone over existing concrete walkway

[digitalorange]

We have a concrete porch (small, more like a landing) and walkway in front of our house. We would like to change it to flagstone or slate tile. We'd also like to expand the walkway a bit to make it wider.

In order to save cost, our landscaper is suggesting that we put the flagstone or slate tile over the existing porch and walkway. He can build around the existing walkway if we want to widen it. Is there a downside to doing this rather than removing the existing concrete and doing a new construction?

I'm concerned about long term durability and whether the concrete is going to expand/contract beneath the stone to create cracks or some other issues I'm not aware of. Also how's the durability of slate tiles. I heard they may wear and chip and I wouldn't want to have to redo things in a few years.

Thanks for any advice!