Now you're no doubt excited to hear when the servers in the USA and Europe are to be moved. Work on regions with more than one server will take longer, however, as we first intend to change the way Diamonds can be topped up.
In future, you won't only be able to purchase Diamonds for one server, but instead they will be stored centrally on your account for all servers. Each of your characters from within one region will thus be able to use your balance. However, to ensure that everything is properly checked and tested, we don't expect this development to happen before autumn.
from
this post .
i bold/underlined the important parts.
first the dia thing will happen in autumn(or after), /then/ the servers will be merged. with this, we can safely assume the actual merges will not happen until after autumn 2015.