Lift existing roof frame to replace top logs in log home
If you're replacing the roofing materials anyway, I'd strip then entire roof off. You'll have far more working space/ease with everything out of the way. (Think about how much fun it isn't to be threading a new top plate log inside your jacking structure...) As @Michael Karas noted, there's a strong chance you've got rot near the ends of your rafters, so having everything on the ground would make it way easier to fix. You'll still need plenty of help (or a crane), though.