Yes, I've tried Flush Privileges.  I've also tried  completely exiting SQLYog and restarting it.  I've also tried stoppping MySQL and restarting it.  In all cases the results are the same — I can't login using the new username and its password, but I can login using the new username w/ a blank password.
Here's some more info:  After exiting SQLyog, I tried logging in from the mysql cmd line.  As it turns out, in this case as well I cannot login with the new username and its password.  But once again I can login with the new username and a blank password.  So this login anomaly is not restricted to logging in via SQLyog.  It's like MySQL itself is not interpreting the password portion of the user table???
I must be overlooking something simple.
Lenny