Old Spock is very much involved! It's more than him just reminiscing about the old days.
Leonard Nimoy's appearance is the reason they needed an actor with a big resemblence to classic Spock. With the rest of the cast they didn't need to be as strict. I mean, look at Simon Pegg, nothing like James Doohan (Scotty). Zoe Saldana, who's playing Uhura, doesn't look like Nichelle Nichols, even on this poster where she's done up as Uhura. And Chris Pine only looks fleetingly like young Kirk.
They shouldn't fret about the resemblence. I don't think it's important that they're exactly the same characters, just that they ARE good characters acted well.