My understanding is that having a higher conviction cancels mephistos aura. But you must have it before you enter the range of his.
Swapping weapons or auras will negate that advantage. Does this apply even if weapon swap has same aura.
Specifically I’m playing HC and my smiters health pool isn’t large enough to rely on dracs to trigger lifetap yet. I’ll be running with other players so I’m concerned for all involved.
If I have conviction cast on both weapon swaps, will casting lifetap and swapping to my heavens light “recast” conviction, thereby no longer canceling mephistos aura?

LIfe Tap wand cancels the whole event.

"Conviction won't be applied to a hostile target supplying higher level Conviction, although this does not prevent it from being applied to other targets."

Unless you are trying to lower the resistance of the Ubers because your teammates are casters, a point in Salvation is usually better.