Can you alter a BIG-IP APM currently using Dynamic webtops to start using Storefront instead?

Bit late to the party, been away. So the issue is if the configuration is in place already, especially iApp related then its not ameanable to being modified. If you do it will rebuild the entire thing. While this is great for dynamic deployment its not so great for production services.

As Jason suggested copy the config and deploy it elsewhere with a different listener address to tinker. Preferably a VE image, until you are sure the components look correct and the inbound and egress endpoint look ok. You should be able to test against that so see if things behave as expected. Once you are happy you can tear down the old and re-deploy in a staging environment to validate it all works before final prod deployment.