Does anybody know how Deoxys will be in HG/SS? I know in Diamond you can choose what form you want but is this the option in HG/SS? Or will Deoxys be locked into whatever form it was in when migrated?
You'll find a decent explanation on how to change Deoxys' form as well as other Pokémon such as Rotom and more here: http://www.serebii.net/heartgoldsoulsilver/forms.shtml. It's pretty sufficient and answers your question well, as does the given video.
For any more questions, please do use the HG/SS Help Thread stickied above above this section's board.