Is Bitcoin's Economic Majority those who already own coins or those who will buy or keep coins?
Each entity's weight in the economic majority is closely related to its ability to devalue the coins in the protocol version they are against.

People (and their proxies) who are willing to offer goods or services (including traditional currencies) for Bitcoin can refuse to accept coins of the "wrong" protocol, while people who hold bitcoins can sell coins of the wrong protocol.

A detailed analysis might reveal the impact each of these groups has, though I suspect the former group has an advantage.

(Personal note: It's been said that I coined the term "economic majority" in this context. When I originally used it I meant the former group.)