Well, I ordered HALO:MCC bit later. It was on 10th December. Sadly, because of the post that came late and some issues, I received it on December 20th.
YES! ONE DAY TOO LATE or I would have been getting ODST update for free.
I still have receipt (bought it via shop on eBay), but yeah… Post screwed up and I gotten it too late.
Would it count if I could prove that I bought the game earlier, so I’d still be able to get this free update or won’t it work and I’ll have to pay for the update? 
SOryy, but the answer is no. Simply buying the game doesn’t count, you also have to have played at least 1 online game (and have it registered in your service record) any time from launch up to December 19th to qualify for the compensation.
Also 343 have released no details as to how ODST will be made available or how much it would cost, if they did release it as paid DLC.
Thanks for answering and damn… Then I’ll have to just pay for it, I guess. Pity.
It’s always possible that they will amend the requirements, but we haven’t heard anything since Bonnie’s blog post back in December. Only thing we can do at the moment is go by what’s been said already. Hopefully we’ll hear more details in the next 2 weeks about what’s happening 
Sorry UDuckling! I am pretty sure it will be worth it to buy if it is remastered like Halo 2 was. Halo 2 Anniversary is beautiful. Cheers