Switching between ADSL and cable broadband providers

[zeus]

I am planning to change my internet provider as I find I pay more for the service I am not satisfied with and I think I can get cheaper, better broadband deal. In fact I like to switch to Virgin Media but I am currently with an ADSL broadband ISP.

Now, how to switch between ADSL and Cable providers? How much downtime will be there while switching? Heard that there could be problems during the switching process. If so, how to avoid those problems?

Thanks in advance,