I have a Phyrexian Dreadnought in hand and in the graveyard.
I also have a Goblin Welder and Lotus Petal in play.

If I play my Dreadnought from my hand, can I use Goblin Welder's ability to swap the in play Petal with the Dreadnought in my graveyard, in order to sac it for the trigger cost of the Dreadnought entering play from my hand?