There is no requirement to replace the LNB unless you plan on getting a dual tuner receiver (e.g. sky plus) and do not currently have a quad LNB.
For BBCHD then any old HD receiver will do.
For ITVHD then you will need one of the new freesat receivers. Unless something has changed in the past few days, SkyHD boxes are not capable of receiving the ITVHD stream, whether this changes in the future remains to be seen