You might need to add sand to level it all out. If you do this, make sure you get masonry sand and not just regular playground sand (although that will work in a pinch too). Just depends on what shakes out under the wood when you pop it up. The wood might be rebar-ed into the ground too, in that case you’ll have to pop it up with a crowbar. That’s what’s nice about wood, is it’s malleable and simple to level out. You might be getting yourself into quite a project with the concrete, depends on what you observe/how fussy you are.