If you have to look at the fingerboard, you're holding it wrong, even while seated. You should be holding it so that you see the board "edge on," with a slight slant in your line of sight so you can see the 1st string through the 6th string, but not like chord charts show it. When you stand, most straps hold the guitar more perfectly vertical, instead of "leaned back" like when sitting. If you have a tummy, you'll end up with even more of an "angle problem," (think of someone like Burl Ives, or Mac Wiseman).

Anyway, practice changing chords as a whole, not finger-by-finger, and without looking any more than you need to. Practice looking edge-on, even while seated. And practice playing, even seated, with the strap on, so that the standing posture isn't significantly different from the seated posture. I always used to tell students to set their strap to the same position as when seated, rather than really low or high.

I hope this helps.

Bob Clayton