I have an Entity Framework designer file (edmx) which has a connection string pointing to my web.config. That works fine.
If I change the connection string I see the changes when the web site is running, but changing the connection string and updating my edmx designer results in using the originally specified connection string.
Upon further investigation, viewing the designer properties shows that it is using a connection string from my web.config, but doesn’t reflect any values I’ve changed in the web.config.
I found that one of the existing connection strings in the web.config was caused this issue. Very strange. It happened to be a connection string that wasn’t in use anymore so removing it fixed the problem.