Reforging item using essence help

Hi everyone,

This essence

"Upgrades a normal item to rare or reforges a rare item, guaranteeing one property"


So may I ask if I were to use this essence on a rare item with 5 mods, would it just add that guaranteed property to the 5 modded item to become 6 mod item OR would it just re-roll the initial 5 modded rare item to a completely new rare but at the same time add that guaranteed mod to it?

thanks

It's just an alchemy or chaos orb with one guaranteed mod. It's not like an exalt.

Ok thank you very much