Agreed. No matter how well you think you prepared, something will almost always cause a noticable problem. Usually that would be DNS caches on the ISP side. As for the actual transfering of data, that really depends on what type of setup you have. Like gate2vn already asked.. Are you talking about a dedicated server, or a reseller account? Do you provide DNS services? Etc...
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Hi...tapan ...that will not create any problem just check that your new server to which you are copying your accounts do not have the same username as in your old server.
I have tried transferring maximum 20 accounts at once time...without any problem, just do this when the load on the server is normal.