This brings up a couple of issues. First, your AVR can adjust the levels so that all speakers are calibrated to the proper relative volume to each other. This can be done automatically or manually with an SPL meter. Second, there is relatively little audio signal sent to the surrounds in most movie sources. Its mostly ambient effects with some louder content for special effect. You can easily get away with having less competent speakers in the surround locations, especially surround rear. The only reason to really pay close attention to matching fronts and surrounds is if you listen to multichannel music, like SACD or DVD-A.
The only real caveat here is to be sure that your center speaker is closely timbre matched to your mains. If your new tower speakers are from the same manufacturer and product line as your center, you have no worries there. If the new towers are completely different, you might consider getting a matched set of fronts and center for the new soundstage. In that case, you still have nothing to worry about using your former mains as surrounds.
