when i replace a base i cut glue through with a craft knife all the way around the base glass, then cut diagonals and tap inside of cuts with end of glass cutter to crack the base
then lever out cut pieces with the blade, being careful to not put too much pressure on the good panes left
then clean all old glue off and follow what evil said
try glass repair places for a replacement sheet, local guy has been giving me 10mm for a good price so i have been using that for replacing bases
shop around