No reason to hide it since your insurance company will change the address on the Dec page or insurance cards once renewal comes in. If you can keep your old number is good to do that and if not have it forward to your new number for about a year or so by that time they will get used to the new number or something.
I think that is what I would do. Port the old so its still available, AND get a 1800. Just add that new # to your list of contact info along with the old #. You can eventually phase out the old # most likely with no issues, and just keep the 1800.