Layer 7 Application Routing?

Okay then. Since all of the applications live under different URIs of the same host. Example:

www.example.com/app1
www.example.com/app2
www.example.com/app3

You could build a data group that maps URIs to pools. Example:

Datagroup (ex. my_uri_dg):

"/app1" := "app1_2003_pool"
"/app2" := "app2_2003_pool"
"/app3" := "app3_2003_pool"
"/app4" := "app4_2003_pool"
"/app5" := "app5_2003_pool"

And then an iRule like this:

when HTTP_REQUEST {
    if { [class match [string tolower [HTTP::uri]] starts_with my_uri_dg] } {
        pool [class match -value [string tolower [HTTP::uri]] starts_with my_uri_dg]
    }
}

You'll need to create a separate pool for each application, and then when you want to upgrade an app, simply create the new pool and modify the datagroup entry.