If you choose the traditional dining (early or late), you choose that for the whole cruise.
And if you choose "personal choice", it's for the whole cruise.
But you can change your mind from traditional to personal choice once you're on the ship...the usual way...going to the dining room and talking to the people there.
I've not done the personal choice yet -- I will be sailing at the end of August on the Island Princess, and am considering it. We'll be a party of four -- and I'm hoping that might mean that if the four of us show up for dinner together, we'll get seated by ourselves, rather than at a larger table.
As CruiseAlholic suggested -- with the personal choice thing, not only do you have to introduce yourself to new tablemates every night, but you'd also have different servers. If you like having the very personalized service that the traditional dining offers, then this might not be a good choice for you.
You won't get very far into Alaska, sailing and returning to San Francisco, I wouldn't think. What -- two days up to Vancouver, then partway up the coast, and you'd be turning around and heading back, on a 10 day. I'd prefer the 7 day minimum, Vancouver to Seward, or back, myself (which is what I'm doing in August.).