I personally believed it would be something that would be announced in their first budget, so maybe you're right they want to do it then. Not holding out any hope for it though.
They've hinted so many times that they're going to drop it early in the Parliament, that I'd be stunned if it doesn't go in the first or second budgets.
I suspect they're quite happy to be able to have a show of strength early, making the point about not spending money. It'll probably make it easier to spend later too, if this makes an impression, as they can file this under "tough decisions".
I like John McDonnell, but it's just showmanship, as he knows full well the reasons why they're holding off making the change. By not doing it now, Labour will hope they have a better chance of staying in power for more than 5 years, and that's going to make a much bigger difference.