Fortunately, it does not appear as though the transfer will be too disruptive to current players. Characters, levels, gear, servers and friends lists will remain intact, and no new client download is required. Mythic will also be hosting German and French-specific servers for any wishing to play in a particular language environment. However, all players are asked to register for a new Mythic account, and will need to make sure that their payment plans and options are set up accordingly in it.
Mythic's also stated that they have long-term plans to allow players to transfer to and from North American servers, although no decision's been made regarding transferring to non-English speaking serves. The transfer begins tomorrow, June 29th, and will continue through July 7th. For the rest of the details, or just to settle your mind, read through the Takeback FAQ over at Warhammer Online's site.