Sounds like you have a dremel tool.
Cut through the the brace at the very corners; all the way through the plastic, so that you basically end up with 4 mitered sections that now have gaps in the corners. Be extra careful as you approach the glass. You're not cutting through anything tough so take a few extra minutes and essentially nibble your way through it.
Once you're left with 4 free-standing sections of trim, shove the stiffest bladed tool you have between the front glass and the trim as far up as you can get it and pry back gently. A 1 inch spackling knife works great. As you pry back, you'll hear the silicone start to peel away underneath. Be patient and work your way from one end to the other. May take two passes. Once you can wobble the trim with one hand you should be able to grab the trim from the top with pliers and pull straight up.
Cut off the other cross brace before you start.
Good luck, and just be patient with it. The toughest part is getting the old silicone off. Be careful with the exposed edges. In some cases the corners aren't knocked down and are sharp enough to cut you good.
Dry-fit the new brace first and makes sure it won't bind anywhere. You'll tank me for this one.
When you're ready to put the new trim back on, run the silicone in the channel of the new one and push it into place. Running silicone on the top edge of the tank glass takes a really steady hand and that's not easy to do with a 6 footer and a manual caulking gun. Be generous with the size of the silicone bead you put in there. Besides holding the trim in place, it also prevents rim leaks which can be a real PITA.