If 51% nodes started to process 2 MB blocks, then it will hard fork the blockchain. It will be a serious issue then, then many got forks by using the wrong chain. Many of the transactions gets processed may not be shown in the blockchain. They have to officially announce the hard fork and make sure all nodes are updated to the change in the block size.
No. Nothing will happen if 51% of nodes support larger blocks. Problems only happen when miners do that. Please read the posts previous to yours before posting.