Drop out rate of an A/B test

We are conducting an A/B testing of an web app, and in the logs we shows who is using the new page B, and who has decided to switch back to the old page A.

How do I construct a search in Kibana to find out the drop out rate of who has gone from B to A?

Thanks in advance!

Could you provide a sample of the logs you are generating? Thanks!

Tue Jun 14 17:32:19 2016 GMT rails 21943-64b2d319-e0ab-47ec-9cdb-09de839bb835 [INFO] (null):0 (null)(): (null): testuser01 has moved to the new CALP, testuser01=NEW_CALP on 2016-06-14T17:32:19+00:00.
Tue Jun 14 17:45:47 2016 GMT rails 28036-ef279ce9-b217-4fd0-94f2-3588a99b0768 [INFO] (null):0 (null)(): (null): testuser01 has switched back to the old CALP, testuser01=OLD_CALP on 2016-06-14T17:45:47+00:00.

Thanks Lukas