The down side is that players on those servers will be experiencing several days of downtime; players who had characters on those worlds before today and have an active subscription will be compensated for lost play time, which is expected to be anywhere between three to five days. Players on those servers will also be given the option to transfer off for free. You can check out all of the the details on the official site, but the practical upshot will hopefully be a more pleasant play experience for European players.
Source: Official Site