The config file should support a "net.http.port" option: https://github.com/mongodb/mongo/blob/r2.6.0-rc1/src/mongo/db/server_options_helpers.cpp#L220

But currently it is not supported and does not do anything: https://github.com/mongodb/mongo/blob/r2.6.0-rc1/src/mongo/db/server_options_helpers.cpp#L407

In previous versions there has been no way to change this port, so this is not a regression. There should probably be a corresponding command line option.