During the promotion of a BDC to a PDC, you may receive:
Promoting <servername> will also close client connections to <BDC servername> and to the current domain controller (if any).
This simply isn't true. Client connections made prior to the promotion are maintained. The clients still have access to existing resources and can establish connections to new ones.
If you were not connected, you must wait until the NetLogon Services have restarted.