during the expansion everything works as usual, so there is no difference in your income during the works on your arena.
to be a little more specific: you can fill the seats you have when the expansion starts while you are expanding, the new seats (the expansion itself) will be available when the expansion ended. eg you have 5000 and expand with 500, then you can fill the 5000 until the expansion ended completley, and then you can fill 5500.
Note that the estimate is an estimate, so if it is estimated 6 days, don't be surprised it will take longer. I've read about prety long delays. There is no reason to panic or report a bug, I've even heard that the estimate keeps being the same for several days before going further down...
They are not your friends; they dispise you. I am the only one you can count on. Trust me.