Is it really as easy as unscrewing the bad one and screwing the new one (with Teflon tape) in? I assume I do this with the pump off. I'm not going to blow up the filter am I?

Further, do I need to know anything to size it when I go to the store for a replacement?

Mine is located in line between the multiport valve and the pipe leading into the filter. The needle rests on the peg no matter whether the pump is on or off.