doesnt need 50% of nodes.
august 1st proved that the protocol can change without 95% of miner support and without 50% of node support.

simply mandating a date that blocks will be rejected on august 1st was enough to change the protocol even when pools did not even update their software. they were blindly told all they had to do was change a 'message' in a version number to act like compliance. and bam. the network changed even though the network was not at the time using the new rules.

for instance after 2 years of promoting segwit. and 10 months since the mandated date. BTCC pool is stil too afraid to put its block reward on segwit keys
https://blockchain.info/block/000000000000000000213def61fb6432460b24ac109f6c8c7276b3efab31c251