I do not know if the International Astronomical Union's commission on naming has decided how to handle this yet. So far, the discovery of the new extra- solar planets has not led to any consensus so far as I know. Since we will be discovering the 'big planets' first, it seems unlikely that a convention such as 'Sol-III' for the Earth will be carried over. Jupiter would be discovered first, so 'Sol-I' = Jupiter, 'Sol-II' = Saturn and so on. It could get very confusing, especially since 'Sol-III' = Earth and naming by roman numeral based on distance has caught-on thanks to prevalent science fiction literature! Probably the convention used for binary stars will be used, which is roughly based on the detectability of the companion object, 'Sol-A', 'Sol-B' etc.
We will just have to wait and see until the IAU feels compelled by events to set up a standards commission and work this out.