If you are converting a freshman Punter to Kicker, or vice versa, his Strength will remain unchanged but he will lose approximately 15% of his GI and Technique. Formation IQ is not impacted.
Work ethic will actually increase if you have one or fewer players already on the roster at the player's new position.
If you are coverting an older player, then the loss at GI and Tech becomes more severe, though Strength and F.IQ still remain unchanged.
If I am taking over a new team and I inherit multiple Kickers and zero Punters - or vice versa - I almost always convert the extra Kicker/Punter to the other empty specialist position because their Formation IQ advantage over a new recruit at that position is usually significant enough to help reduce the return TDs that they would give up. I might also recruit a new Kicker/Punter who will eventually be better than the converted player, but at least my recruit won't have to play as a freshman and won't give up all those return TDs. And my new recruit will still see their Work Ethic grow, even though they aren't playing, as long as they are #2 on the depth chart. That's a lesser-known feature of being in the Top 2 on the depth chart.