It sounds like you need to setup multi-bitrate streaming. It is all covered in articles from Adobe. See the links at the bottom of this post:
http://community.wowza.com/t/-/65
The tricky part is to find an encoder that produces properly aligned streams. The only options I know that work at this point in time are: FMLE (read EULA), Digital Rapids, Haivision, Inlet. Many other encoders will not work because the key frames are not aligned.
Any interem solution using a non-aligned encoder is not really supported in Wowza.
Dual-threshold buffering will not work with a live stream. There is no source of video for the second buffer.
Charlie