hellsing,
you should read the 3GPP documents I pointed out earlier ^^
3GPP 23.060
http://www.3gpp.org/ftp/Specs/html-info/23060.htm
6.9.1.2.2 Inter SGSN Routeing Area Update
The new SGSN sends SGSN Context Request (old RAI, TLLI, old P TMSI Signature, New SGSN Address) to the old SGSN to get the MM and PDP contexts for the MS.