If I read your post correctly you have a toilet with the ball valve that rotates 90 degrees to allow it to dump.
If so the change is not difficult at all.
First thing to do is make sure the water is off. (Of course the toilet being clean helps

)
Then you look under the toilet you will see a "Cover" that has two parts to it. You unscrew the screws holding it together. Some the cover simply slides outward.
After removing the clamshell cover you will see between the bowl and base what looks like a screw clamp simulat to a radiator hose on your truck/car. Loosen this screw clamp. The bowl will then be able to "Twist or turn side to side". You then can twist the bowl so that you can undo the water inlet.
This inlet is under a little cover at the reat of the toilet bowl on top just behind the lid. Remove the screw and the cover will come off. Under the cover is the vacum breaker.
This vacum breaker actually just slides into a rubber grommet in the rear of the toilet bowl. Gently work the vacum breaker (Twisting it) a little back and forth while pulling it away from the bowl.
After you get the vacum breaker out loosen the large screw clamp some more until you are able to drop it below the thing it rides on.. Under the large screw clamp is a two piece plastic ring that has a "Wedge" on the bowl side of it. This is what holds the bowl to the base. Pull it apart and set it aside.
You now are where ya want to be!
Lift the bowl up and majically you will see the seal. It sits on top of the ball valve.
Clean the sealing areas of the ball valve, base and bowl. (I usually spray some silicone lube on them too)
Change that sucker and lube it with some silicone lube. (NOT sealer)
Now ya reverse the process and you are Reddy Freddy the toilet repairman.
Sometimes all ya gotta do is clean the seal and lube it to fix the leakdown problem.
When you are finished sit back and hoist a cold one to a job well done.