Remove no-flat tire from wheel

I have two worn out 23x8.50x12 turf tires on my PM-400 Gravely, and want to replace them with pneumatic tires. What is the easiest way to get those tires off the wheels? (Gravely wants over $200 each for the wheels, and they are different than all others I've found.)

Explain what you mean by no flat?? Are they foam filled or are they some sort of seal them self tires or what?

I have never cut that large a tire off, but here is what I do on 6 inch 'no-flat' front tires if I want to save the rim:

Be sure the rim is capable of mounting a conventional pneumatic tire before you start. Some of the no-flat rims don't have the correct bead for a pneumatic tire to seat against, even if you do drill the valve stem hole in them.

There is an air pocket in all of the no flat tires that I have worked on.

Using a utility knife (The best utility knife is a Stanley that doesn't fold up or retract. Don't use a cheapo as the blade can pop through the joint in the halves and cut you really badly.)

Make 2 parallel cuts across the tread with a utility knife about 4 apart. Start about 1 inch from the rim, go across the tread to about 1 inch from the rim on the other side. I fasten the rim down and use taps from a small ball peen hammer to help force the blade through the rubber. Make multiple passes with your knife.

Connect the two cuts with cuts at each end and break out the piece of rubber that you have cut all the way around. Then make one cut as deep as you can go through the middle or inner part of the rubber in the middle of the 4 inch wide hole you just made. When you get close to having it cut through, or if your blade is at full depth, you can use a couple of pry bars to break it the rest of the way and then pry the rubber tire off the rim.

Keep in mind that one slip and the utility knife can cut you really badly.

I'm not familiar with your model, but have been a Gravely fan for decades. From what I can find on the internet your mower seems to use the Gravely 20928200 wheel. This wheel seems to have been widely used on the Gravely Professional series riding mowers as well as a lot of other machines. It's rather a pain to do a full investigation because the Gravely seems to identify what a part fits by numerical model numbers--not the model names on the hood decals. Also most part numbers were changed when Ariens acquired Gravely, and some have been changed several times since.

https://www.jackssmallengines.com/jacks-parts-lookup/part/gravely/20928200

Depending on where you are located, it may be easy to find these wheels used at minimal price.