Originally for solo flute and string orchestra, the flute part is given here (transposed down one or two octaves) to both the viola and the cello. At each repeat, the solo part changes between the two instruments for a perfectly democratic distribution of both the difficulties and the protagonism. The easier version is simply transposed down one tone (into A minor) but apart from that it is identical to the “harder” version, and is, in fact not that much easier.
If one player finds the solo part too difficult then he/she should just always play the continuo (simpler) line.
Bach starts the piece with an upbeat. In my arrangement I have taken the liberty of displacing the beat (the upbeat is now the downbeat) so as to make it read as it actually sounds. My apologies to Bach.