All these models losing options really annoys me, since I spent a lot of time and effort converting and making models. Especially for models that have had these things as options for a long time, rules-wise.
I believe the official ruling from GW is that any units/options that existed in the Index books that is not listed in the codex may still be taken using the Index entry. For example, Roughriders are apparently not in the new IG codex (so I've read -- I don't own the IG book, so I can't say for sure), but IG players are still allowed to field Roughriders using the entry from Index Imperium II. Likewise, Marine players who lost Dreadnought or Razorback weapon options can still use the Index Imperium I entry for those units to get their desired options back.
It's a really messy state of affairs, and I fully expect that once GW gets the initial dump of books out of the way and gets back to making plastic toys we'll see models packaged with datasheets with their appropriate rules, giving you a lovely stack of books, pamphlets, printed FAQs, and eventually Chapter Approved books to haul around.
Things will then get slightly neater with the release of Chapter Approved for a few months, while new releases continue to produce new model-specific datasheets, which will then be collated in the next Chapter Approved book.