Edited the info in the dat and it worked.
We are currently working on some enhancements to make preferred server work much better. The patch is almost ready for testing. Stay tuned.
Some enhancements include:
TTL on each client for preferred servers.dat will be shorter
Peer download for the file will be turned off (so a potentially old file cannot be used)
caching info on the core side to speed up web service requests (which will make changes to configuration take effect much sooner)