The base damage type of specific skills can be converted. For example, a Physical skill could be converted to a Fire skill. Conversions are typically found in skill trees and items, and sometimes in passive trees.
Base damage conversion converts a skill’s base damage from one type to another, which does not affect damage from stats such as "+5 Melee Fire Damage", but will change the damage type of added damage stats without a damage type tag such as "+5 Melee Damage".
Damage conversion doesn't affect ailments such as ignite, or any other effects applied or triggered by the skill, unless stated otherwise.