You will firstly need to break up the bottom of the pond to allow drainage. It is nice to think that you can just throw in large chunks of rubble but these will form big void spaces. As the years go by the soil from the top will filter down to fill these void spaces and the surface will sink. (Been there, done it, got the 't' shirt etc). It is worth spending the effort to break the rubble down into small pieces first.

If you want to change your pond into a lawn you will need to remove, or seriously pierce/break up the liner otherwise you will have stagnant water sitting below you lawn. Te snag with filling with rubble is the soil will settle and become uneven resulting in the lawn being less than smooth. Unless you really want a pond there it would be easier to create a bog garden.

if you are going to get rid of the pond then it would be worth the effort to remove the pond liner totally. then you have the best finish in the long run. in the future the difference in the drainage may affect how well the grass grows.

you considerd a bog garden it would be ideal.