You're making a heck of a lot of presumptions! Firstly there is no question of this being a "...time line alternate universe type story...". None whatsoever. As for your reasoning for this assumption...
...again there has been no mention of this by Abrams or anyone connected with the movie, it is not addressed in any early drafts of the script that have been circulated, and is simply not an issue. There will simply be new actors in the roles. No explanation for different appearances necessary. Audiences will be able to cope with this I feel. Think Alec Guinness - Ewan MacGregor.
Once again, Alec Guinness - Ewan MacGregor.
Once again, where are your grounds for these assumptions? Indeed from what we know so far, canon and continuity are uppermost in Abrams' mind. There is nothing whatsoever to suggest that anything is about to be tampered with. Simply because new actors are involved does not mean the basic ST timeline / canon will be changed.
I find this bizzarre and somewhat reminiscent of the irrational furore and wild predictions and assumptions over Daniel Craig's casting as 007, "James Blonde", "boycottcraig.com", etc. Not to mention a similar getting-of-knickers-in-a-twist by Trekkies when
ST:TNG was announced - threatened boycotts of the show, "No Trek without Kirk & Spock" letter campaigns, etc.
And look how they both turned out.