Technique differs almost completely. Knowledge, however, doesn't. If you understand some theory and have applied it to bass guitar, know how basslines work under chords, what notes to highlight, etc. (aka the bass's role) the only thing you really have to worry about is the new technique. You won't go through the whole mess that beginning musicians must learn, but just the technique of the instrument, and with your experience it should go quicker then a beginner. It doesn't matter how much different upright is from electric --- you could be learning oboe, the fact that you understand how the muscle coordination works is a huge plus.
I had played bass guitar for years, along with classical guitar, electric and acoustic guitar, and lute. 2 months ago I got an upright because I love bass and it is a very marketable instrument. Things are now coming together, I can play coherently, in pitch, loud enough, and with decent tone. Now I only play electric bass if it is necessary (like if I'm playing for a rock musical, which I am doing next month). I am sure that if you get into upright, you will fall in love with it and not want to go back (this is a positive, not a negative!).
Best of luck.
Alan