I need to change my domain's DNS, but here in Thailand the ISPs don't follow any standards for renewing their cache of records, so it can take the full two days.

I have a new website with about 1,000 members currently who are sending messages and uploading photos. I don't want both sites on the old and new server running at the same time because then I'd have to manually merge the database and transfer uploaded photos. It would be a huge pain.

Both servers also a unique URL from my host, e.g. website12345.hostingcompany.com (old host) and website67890.hostingcompany.com (new host).

I don't have much experience with this, but I think what I can do is on www.mysite.com, use .htaccess to do a masked redirect to the new server's URL (website67890.hostingcompany.com). Is it possible to do this and keep all URLs being masked? For example, www.mysite.com/profile/username will actually be loading website12345.hostingcompany.com/profile/username. From Google searches it sounds like this is possible, but I don't understand why this is possible due to security issues, since what's keeping people from masking their site to URLs like facebook.com?

I could really use some advice here! Thanks!