Hi there.
As from the name of the topic, my proposal is quiet clear and let me explain why. 
Since in 1.1.4.2 Soulrend got purely cold damage base, we have no phys damage to convert on it.
After that, the weapon is mainly used by Spellbreakers (dunno if used still) and Reapers - class combos, including Nightblade mastery - which has no phys damage in line of its skills but has lots of pierce.
Then, the main skills used with Soulrend are PB or BH - again with pierce base and with no word of phys.
So, changing the conversion will turn to more damage and slightly improved QoL for these builds and increase build diversity instead of leaving the conversion pretty useless.
Edit: also, adding the acid/vit to pierce conversion on Soulrend will look nice too, as said below. Moreover, it will also help to make some nice concepts.