To shift everything into the cloud you have to address all those bits of legacy pipework you have in your systems. You have to understand all those strange data flows and interdependencies that exist and will break if you move the wrong piece first or in fact any of the pieces in the wrong order.
You have to be clear on your scope and what work you won't do while you are migrating. Some of that might be a pause on additional business features while it takes place.
In some cases you will have to deprecate or find a replacement for technology you already have because what you have already isn't cloud friendly. This can be serendipitous as per the cost cutting horror in the Unicorn Project.
Then there is "everything" for a given value of "everything". Does everything include cloud based IDEs where your workstation is basically a browser?