Form changing and not being needed for the National Dex have no connection.
A showcase of this is Kyurem, Giratina, Cherim and the Genie Pokemon, who have different forms in BW but are needed to complete the dex. This means that changing form hold no relevance when determining if the Pokemon is needed to complete the Dex.
So what determines being needed for the Dex? What factor does the Gen 5 Event Legendaries share with the other Pokemon that aren't needed for the Dex? Simply putting it, it's their status as Event Legendaries. Normal Legendaries are still needed for the Dex, and so are normal Pokemon distributed from events (although the it's just the specie that matter, not the fact that it's an event Pokemon) but Event Legendaries like Mew, Deoxys or Arceus aren't necessary to complete the Dex.
Long story short: Form Chaning have no impact on being needed for the Dex or not. The only relevant factor for determining this is Legendary Event Exclusivity.