Or better yet: Cut the threads near the "old" spools, switch spools. DO NOT PULL THREAD OUT. Tie knots in "old" thread ends still threaded thru tension, etc. with thread ends from "new" spools using square knot. Write down tension settings, turn tension to zero. Pull threads thru. You may have to cut thread at needles if knots do not go thru holes in needles, if so, rethread needle making sure you use correct thread for the needle. (Upper left tension with upper left needle, etc) Reset tension settings, sew.