Hi Paul,
As koenning pointed out. Any existing TCP Sessions would continue using the cached (old) iRule until their TCP Session has completed.
Any new connections would cache and use the new iRule.
I would be very careful with the b conn command. If you executed the "b conn all delete" this would delete / reset all connections to the entire device which would be highly disruptive to your traffic.
If you need to use this command you can get more granular.
Show connections to specific Virtual Server:
b conn server
virtual.server.ip.address show
Delete connections to specific Virtual Server:
b conn server
virtual.server.ip.address delete
The graceful solution is to either allow users to fall off by attrition and get the new iRule when they come back, or direct them to close out their browsers / sever their connection and then come back in a new browser.
Hope this helps.