There asre several schools of thought on removing glued tires.I have heard all sorts of stuff but I will tell you what I do.
It sounds like you were able to get one side off – your half way there.Start working on the side that is giving you trouble by using your thumbs and rolling the tire sidewall down and toward the center.You can use some pressure here (I don’t think your thumbs will be strong enough to snap the plastic rim).If you cannot get the rubber to peel away from the rim you can use a blade to carefully cut between the rim and the tire.Be careful not to cut into the plasticof the rim (it’s okay to cut the tire because your going to trash it any how.)After you get the tire off the rim you can use your blade and some sand paper to remove the left over glue on the rim.
When you replace the tire, use a few drops of CA glue (I use thin CA and let it wick in) between the rim and the tire.Put the tire on the rim BEFORE adding the CA glue.If you don’t glue the tire to the rim 2 things will happen; the rim will spin independently of the tire and the tire will come part way or all the way off the rim during operation.
Good Luck and have fun!