Simply put, you don't want to stress anything. If your cuts are not complete, don't pry or force. Much better to cut the battery tabs than mangle the traces on the motherboard.
As already pointed out, if you want a new battery, get one with leads so you can mount it away from the MB. You will have to solder the new ones on, red to +, black to -. Rechargable phone batteries work well. You can read voltage and capacity (in mA/Hours) of the original from it's body. Your new battery should be rated for the same voltage. The capacity should be at least equal to or greater than (can be several times more) the original.
Gee, we've nearly recreated the procedure again. Grin.
