Actually, that's a very interesting question. From a technical standpoint, you only need the consent of the active miners. From a practical standpoint, you mostly need the consent of those managing the reference client.

Theoretically, a disagreement between a large group of miners and those maintaining the reference client could lead to a fork where the public hash chain splits into two incompatible groups of programs, each rejecting the other's hash chain, where everyone with bitcoins prior to the split has them in both systems (and could spend them differently in each system). However, letting that happen is in nobody's interest. So it's extremely unlikely.
In the post you are replying to, which you already read.