Go and have a look at
www.frequentflyer.com.au
This solution won't keep them happy. I've never seen a worse bunch of whingers in my life, quite frankly.
As for this solution, I think a combination of increased bonuses and retaining a portion of old points is needed. Not giving back anything may be enough to keep pax away once and for all (I mean why bother, once you've started earning QF points, for example) - what's the point of starting all over again.
I hope they're clever with their program.
Cheers,
Justin