I presume it's an EXO-45, 5K-8 ohms. Ideally of course, in that case, you would sell those and get some EXO-46's which are 5K:16 ohms.
There are impedance-changing transformers and autoformers on the market that have taps for input and output of 4-8-16 ohms; those can be (for instance) installed at the speaker. But it does add another transformer into the signal path.
There is nothing technically wrong with driving the16 ohm speaker from an 8 ohm tap; you lose a couple dB of power but gain slightly better damping factor. Given the low impedance of the stock tweeter, a 16-ohm transformer would change the timbre of the speaker a bit, making it less bright. Still, it's not a big difference.