Well, it all depends how you define "upgrade." Jake Cave started 38 games in LF last year. Schwarber 103. There are plenty of part-time players out there, even for $5 million, who can probably combine with either Pache or Marsh to be more productive overall (Schwarber's offensive value was practically negated by his defensive play, plus he didn't really get going at the plate for good until he was off his feet.)
I think one way or another your starting CF is either Rojas or Pache, at least 50-60% of the time, with Marsh the rest of the time. So you just need a LF to share the rest of that time with Marsh.
If they decide Rojas is going to spend the entire season in the majors, or be more of a 4th OF for the majority of this season, that's a different conversation.
I think a trade for an OF is also possible. My guess is the Castellanos dream is already over. The floated it, nobody nibbled, and then they denied it to make Nick feel better. Sure, it's still possible but it was always, and remains, unlikely.