Can't see anything in the rules either way about this, but can you change your fleet's objectives during the refit stage? It would seem odd to be stuck with them for the duration of the campaign.

Nope - that's what Skilled Spacers are for (to temporarily swap out one or more of your Objectives for ones better suited to a specific match)

Bugger, because we just started a campaign and I picked the wrong objective.

It's casual, if you've just started, ask the opponents if you can change but if it is a change in direct response to their fleets they may not allow it; if it's just because you made a mistake then they might.

Other than that you'd best try and take a planet with Skilled Spacers!

Or only attack/defend on planets where your dodgy objective won't come into play.

To kind of piggyback on this thread; can you use skilled spacers to change out one of the green campaign objectives like asteroids or one of the base defense objectives?

No - they are strictly for your Objectives only. They change the Objectives you add to the green Objectives.

Good to know. Thanks!