APM Policy Selection Based on URI

First, did you create this configuration with the iApp? You may have missed some steps. The iRule should definitely support your use case.

In lieu of that, or if you're doing something different than the iApp-based deployment, you can technically switch between access policies, but you have to do so through a "layered" VIP architecture. Essentially, you have an LTM VIP in front of two internal APM VIPs that terminates the client side SSL and sports an iRule that switches between the internal APM VIPs using the virtual command.

Before taking that path though, please review your configuration and/or build a separate iApp-based config and compare the two.