In set, costing 3 converters, converts to a random IO within the same set. Anything except for the one you start with. So yes, it will convert to a different Thunderstrike, with a 20% chance of getting any one of the other five.
In category (you have to select this from the pull down menu), costing 2 converters, converts to a random IO of the level you started with in the same category but of a different set. For example, a Doctored Wounds can convert to a Miracle, but not to another Doctored Wounds.
By rarity, costing 1 converter, converts to a random IO at the same level but of a different set.
Hope this helps! If there are any follow up questions, have at it!