Those impedances seem really low for tweeters, they typically range in the 30's to 100 ohm ranges
that aside, you should be able to use the 8 ohm new ones and just play with the equalizer a little, its going to draw less power than the old ones, so you shouldnt have to worry about breaking the system, it will just be a little quieter, if you are going to do 1 you should probably do the other as well