Hey Chris - expect the alien-hands-on-electric-bass feeling to continue. I've been playing electric bass since I was 13, upright bass since I was 21 - so whilst I'm more proficient on the electric I somehow manage to get most of my work on my upright, especially original work. I've done several 3-4 weeks tours on the upright and when I come home and pick up the electric it's heartbreaking - 20 years of playing and you play like this?!?!?!?!?!?!
I think it's not only the difference in string spacing but also the timing of your attack that changes. To me an electric responds immediately to your touch, whereas an upright is a little slower as the note develops, so you subconsciously start to accommodate that. Moving back and forth between both - it's like getting your sea legs I imagine.
The other weird thing you might find, coming from the electric world first, is that when I'm gigging the upright with a loud band (drummers/guitars) the upright never sounds solid enough. I think that's because I'm so used to hearing an electric bass in that context. So what I'm saying is that you sometimes have to mentally adapt your expectations of sound and performance to fit the setting.
And whenever you get the feedback blues and the rest of the band are yelling 'can't you make that thing any louder?' - just reply with 'That's why we have electric basses!!!!!'