I live in a bilevel home. The upstairs toilet is directly above the downstairs toilet. When I flush the toilet upstairs,The water in the downstairs toilet splashes all over the place. This only starting happening when I replaced the upstairs toilet with a new Kohler toilet

Sounds like when setting the upstairs toilet, some of the wax ring got squeezed into the drain pipe and is now partially blocking the main drain. Pull downstairs toilet and snake the drain line.

Good guess, Harold. Agree, there may be a clog below the downstairs toilet. Do as Harold advised.

Milo