If its got mold/mildew inhibitors in the silicone it will be bad.
You can use a razor blade to cut out the old glass. If that doesnt work, try an old tapemeasure between it, works great.
To remove all the old residue, youll need some steel wool, a razor blade, 99% isopropyl alcohol, and some microfiber cloths. 0000 steel wool grade is good and wont scratch the glass. First slice off the silicone with the razor. Then scrape it more. Then scrub it hard and dry with the steel wool. It comes off. Then there is a silicone oil residue remaining. Start buffing hard with the microfiber cloth. After you scrubbed it dry, spray it with isopropyl alcohol and scrub again with microfiber cloth. Then rinse with ro water and paper towel it dry of any residue from alcohol. Its now ready to use. Good luck.