What I can help with is to monitor availability of what you'll ask me to monitor.
I can also duplicate my test pages for new location to monitor them just like I monitor them on current server.
Some email to users before the move will be good idea, I think. Especially that they will need to update their DNS settings.
But at the moment something seems to be wrong with your JustHost account. Users websites and their access to CPanel etc. seems to be fine.
It appears justhost has taken it upon themselves to suspend my account because they feel like it. In this case a different domain, on a different account, was hacked and they suspended my main one and sent me an email about it. For what reason.. I cannot fathom..
I am pulling everything off that server as soon as I can. Starting with chemicalservers.com
Unfortunately I have to do this without breaking all other sites.. >.>
So NS1&NS2 will still point to the justhost server until everyone has been moved.