It’s absolutely not too late to make the change. I designed our kitchen with a slide-in range but I would not have designed or installed the cabinets any differently for a freestanding range.

I have included a link to my website showing the cabinets during and after I installed them. You will notice in the top most left photograph the base cabinets probably look similar to yours before the installation of the your countertops. The only difference is a sliver of countertop (we chose granite) installed at the back of the stove closest to the wall. To support this small 1 inch thick I just built a shelf attached to the studs in the wall. The back lip of the stove covers it almost completely and there is hardly any weight on it. The sliver is glued to the countertops over the adjacent 12 inch base cabinets

I like the slide-in since it allowed me to have one long piece of granite backsplash along the wall as seen in the photograph at the very bottom of the page.

If you are choosing formica or any other countertop product, it should not be a problem to just cut out what is not needed when they install the counter. You definitely need to buy and have the slide-in stove in the room before the counter install so they know exactly how to cut around it.

Good luck,