I'm the same way. With the expection of a couple of my reels that have very shallow spools (TD-S 105, Morrum UM Forrells) I remove half the line off the spool and then tie the new line to the old. It's normally between 75-100 yards. This saves time and money when respooling.
Same here. I have been using mono, and if the line starts feeling kinky, or if it looks faded compared to new line out of the box I will put new line on.