Its fixed for 12 months for the cap (I am on the 35p agile with 21p SC and its been honoured even though there is a new agile now), but perhaps wait until October or November to sign up.
Also when you sign up, you can hold it for 30 days as there is a final click to agree which has a 30 day expiry.
You even have a get out of jail if you wait and they pull it off the market, I will be honest on here, I missed the expiry for the old gas tracker, I was already on the old agile, I emailed Octopus and told them I tried to switch on the phone but it never happened, they then put me on the old tracker (v3), if you do this, do it within a week of them pulling it so its believable.