Well, I hate to let go of Jellicent, but without Stealth Rock, I can't really justify holding onto him, and I'm not overly attached to Gyarados. So the direct substitutions are easy enough.
The attack changes I'm also all for, including Mamoswine... which brings up the only problem. I'm really, really attached to the Choice Scarf. It seems weird, but no one expects to get killed by a superfast Mammoth, and he makes an amazing revenge killer to boot. What benefits, specifically, does Life Orb have over Choice Scarf?