I asked the question on Digitaluk and got this answer back - not very - suggests they don't know either - but refers to HD channels plural - is there more than BBC2 HD being launched?
"Thank you for your email dated 19 March regarding retuning after BBC2 HD launch.
Prior to the launch of the new HD channels, you will be advised by on screen messages of what action you will need to take (if any) to receive them.
If you need to retune, we always recommend you select the Default Settings option from the Installation menu."