Interesting article from What Satellite and Digital TV (July 2012, i.e. during DSO):
http://www.dtg.org.uk/dtg/press_releases/dtg_whatsat_201207p20.pdf
It seems auto-retuning is facilitated by the DTG but is optional. Or was then.
And the DTG says:
Network Change Notify descriptors (NCNd)
Network change notify descriptors are used to signal significant platform changes. A start time and duration of the work is given, at the end of which receivers can offer the viewer the opportunity to re-tune and acquire the change. An associated message may be broadcast to inform the viewer of the work prior to the event.
http://www.digitaluk.co.uk/industry/About_DTT/glossary_of_terms?result_76219_result_page=N
From this it appears autotuning is still optional, as far as the DTG is concerned, and Humax could give users the option to disable it.