SF EXTREMA vs SF Cremona M

The common denominator here is that SF enthusiasts want your Extremas and are willing to trade current models to get them. I'd be very cautious about giving them up.

I've heard the Cremona Ms and love them, but based on what I've read, the Extremas have almost another octave of bass extension.

There's someone on Audiogon right now selling the Extremas for 9250$. He's definitely overcharging, but you can see their worth.
http://app.audiogon.com/listings/full-range-sonus-faber-extrema-with-stands-shipping-cartons-beautiful-2013-01-25-speakers-37876-sevierville-tn

Still considering the Elipsa, Extrema option. Cremona is out. Thank you. Hope to gather more advise.

What is it about the Extrema that makes you so eager to trade them?

Elipsa's vs. Extrema's is a very interesting trade. If you need a speaker with more ability in the bottom end, I would go with the Elipsa's. However, not at any kind of uppricing.
OTOH,some of this would depend on the condition of your Extrema's.
You would be trading a speaker with a better mid and high freq driver for a speaker with a more modern design and as previously stated a little more bottom end drive.
Like Donjr asked, what is it about the Extrema's that makes you so eager to trade....IMHO, they are one of the best speakers that SF made.