You are going to want to version you ext file when you deploy.
So instead of having:
You can have:
It would have a cache header that never expires and when you update to another version the name will have changed and it will force the browser to load the new file.
When you upgrade your to Ext 4.2 for example it would be :
Here is a really good write up on deployment level web devlopment. http://developer.yahoo.com/performance/rules.html Make sure to check out the "Add an Expires or a Cache-Control Header" section which talks about versioning. Versioning should also be done with your component/application level scripts as well.
This is a shameless plug but you can check out the source of my website http://www.coffeedig.com/coffee/ which is a django application written in Ext to see the versioning in action.