normally you can shift a song by simple ratios that are in the original key (like how a perfect fifth is considered a 3:2)
Taking that example, we'd adjust the bpm to around 82 (123*(2/3)) and the arp would stay pretty nicely within the key aside from a few notes here and there. That's what I did with the bass in the original track. I stretched it by a 2:3 ratio to take it down a few steps. Did I explain that clear enough?