So what would happen if say 51% of nodes would switch to 2MB blocks? What would be the consequences?
If they simply changed the blocksize limit, this would be a hardfork, so the network will split into two incompatible versions as soon as a >1MB block is mined.
However, a firm softfork (
sample code) achieves the same thing (2MB limit rise) except the network cannot split. The code is a bit more complicated however.