> At least when implemented like BTC Guild, changing the setting in the Web UI instantly applies your setting to any active connections to stratum servers that you already have with that worker, on top of future connections.
That's nice to know. Does BTC guild treat this as a minimum difficulty, and never set the difficulty below it?
Yes. If you authorize a worker on your connection with a difficulty set higher than what it was already running at, it will immediately increase your difficulty to the new minimum (since stratum difficulty is per-connection). It will never drop below that setting for the remainder of the session even if vardiff would normally decrease it.