Boon of Mutation at 2pts is absurdly cheap, and I think the Daemon Prince should retain all of his equipment and Mastery Levels with the exception of Terminator Armour. Apart from that, pretty neat.
I guess you could say his grenades disappear, because his hands are now way too big to use them, but if the Warp can turn a human (well, close enough) into a giant winged demon, then I'm pretty sure it can scale a sword up - he should keep all his gear other than any grenades.About the Prince...you are right!!! I forgot about mastery levels! Argh! But all the equipment could become risky: blight grenades, meltabombs, sigils of corruption...basically a better daemonprince?
I'd say mastery levels, relics and armour save 3+.
OR open up princes to sigils of corruption, familiars, "mortal" equipment?