This is what I have to say to psylockollusses with regards to this 'prequel/sequel' business: in the words of Inigo Montoya, "you keep using that word. I do not think it means what you think it means."
I had previously been convinced that First Class invalidated Origins: Wolverine, but in light of multiple developments since then, have changed my mind. There's actually nothing in FC that does anything whatsoever to O:W, and I don't believe that Singer will do anything in DoFP to invalidate The Last Stand and/or The Wolverine. Things that are currently seen as inconsistencies - such as Xavier having the use of his legs - can be addressed without invalidating anything from the existing films, and things like the Cure being reversed and Xavier returning from the dead are things that were actually seeded by TLS itself and therefore would not count as invalidating that film.
I've seen nothing whatsoever to convince me that whatever 'fixes' Singer chooses to make will in any way alter the composition of the X-Franchise, despite what some fans might want.
IOW, I firmly remain convinced that all of the movies that currently comprise the franchise - FC, X-Men, X2, TLS, O:W, and the as-yet-unreleased TW - will still comprise the franchise when the events of DoFP are completed.