The recommended way to lay astro lawns is to remove existing lawn and soil to a depth of 6", add a geotex membrane then backfill with hardcore, compact that then perhaps add an underlay shock pad followed by the astro itself.
You are going to have to undo all of that, dig out and dispose of the hardcore and buy in decent topsoil, level and plant that. It's a little too late to seed a new lawn now but March to May is a good time to get one going, and that gives you time to plan for, and remove the existing lawn.
It's very DIY-able if you are so inclined, but will be hard work, or back-breaking if the lawn covers a large area. Every ~6 sqm of lawn area will mean a cubic meter of hardcore and material to be removed.