OK, so I did the soft reset of my two 3-channel amp blocks. The #34 required a general +1 dB level change, while the AC30 one needed different level adjustments for the different drive settings - all was within 3 dB, so not that big a difference. On the AC30 I was also tempted to add the slightest bit of bass in the amp block, but I ended up not doing it.
A function where you could select a bunch of presets and then automatically replace the amp (or any other) block with a block from the block library would be really handy. I don't care, if it takes one minute per preset, but yesterday, I updated the amp and cab blocks in 40 presets with blocks from the block library. The worst part is not the long time or the extreme boredom, but there is also a large risk of mistakes. An automated way would be so much better. It would eliminate my need for global blocks.