merc changing malus

[Chameleon]

i have 2 questions:
the less important:

i´m recently using a defiance merc.
i got a pala (shocker/avengerdin) with about, 1600 base defense, when im used holy shield, i get about 4000, if my palas aura s on, im on about 8000

is it good to change to holy frost?
the more important:

im now at lvl 78, my merc is it, too.
when i change my merc right now, how much loose of
a) char attributes
B) merc aura skilllevels
would that be?

i think, it was earlier, that a merc had lvl 1 aura when u hired him and then got
one skilllvl each 3 lvls he went up.

this beeing the case, it would be utter folly to change mercs. anyone knows about it?
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You don't have to look far for correct information.
Spirea's Diablo II Resource

As for merc equipment to make him something more than fast fodder in hell.
Body armor with 1200+ DR and fhr.
Helm: I use Tal Rasha's Horadric Crest on nearly every merc, dr, 15resis all, 10 life leech
Weapon: Only use Hyperion spear, Mancatcher, Thresher or Great Thresher; -10 base speed, the faster you merc attacks, the longer he will live. Honor and Insight are great cheap runewords that give high enough damage to keep him going.

As always with merc item, Ethereal is gold for a merc.