On "Auto", It does switch automatically to the impedance related to the 1st active block in the chain after the input block (Mixers, Volume Blocks, Send Blocks, +other active "utility" type blocks count wrt this (for some reason, an active looper block does not)). The change does not show in AxeEdit for me, but it does on the Axefx front panel in the Input1 block info. Not many blocks will, if positioned as 1st active block, change the impedance to something other than 1M - examples I noticed are the Treble Boost drive, Phaser on Classic Univibe, and some of the Fuzzes.
After a fair amount of auditioning yesterday I've concluded that I can't hear or feel the difference between 1M impedance and low impedance (maybe it's the guitar (sg)? or probably because I just have tin ears and hands). Just as well as I now won't be trying to get certain blocks to the front of the chain - pita depending on the patch requirements, if you just have to have your pickups seeing that 70k on univibe or whatever.