Bill if u have a cool season grass issue, u can use oust to nuke that. I sprayed in fall and it didn't hurt the native grass. It's like having a clear slate for the switch come spring.
I wouldn't count weak spots out at this point. I've had some good chunks look real bad in year 3, and really start coming around year 4. It takes a little effort but this grass will grow. It's really a lot of fun.