That’s not even a real choice to make. Come on now.
I think they decided to go for it at the end of S01, because of this interview Marc G gave back then:
"the final four episodes really are about this love triangle of Oliver, Laurel and Tommy. That’s the love triangle that we began the series with so we felt it appropriate to finish the season with a focus on it. That love triangle really starts to come to a boil with Wednesday’s episode, Episode 20. That kicks us off for the remaining three episodes of the season after that. The truth of the matter is that we sort of have to play that out first before we can play out Oliver and Felicity. But I love the fact that people are shipping them. It really is exciting. There’s nothing but love for Felicity among all the people involved in the show. So, stay tuned. But in the meantime, I gained a lot of satisfaction by teasing people.” (x)
The interview was posted in Apr 24, 2013. As someone who is actually experiencing fully support of the writers, the few interviews I read regarding O/L and comparing them to what Olicity gets is completely different. For example, SA last year at the Comic Con said that even if Oliver believed Laurel was the woman of his life in S01, he’d have other love interests. Now, talking about Olicity, the idea of Oliver being with anyone else while in love with Felicity, it’d undermine what he feels for her. That just sustain my idea that the writers had the idea of O/L go before S01 ended. They didn’t promote the two of them a lot, specially when we see at what point the writers go when they want to sell an idea (“I think they’re in love with each other” - “We’ll deal with that directly this year, their feelings for each other, which I always believe have been genuine” - “Felicity and Oliver actually have a pretty raw discussion about what they mean to each other, which we’re really excited about”).
So, i don’t think it took a lot of time for them to actually let it go. I believe they didn’t show people they were actually letting it go abruptly, so they didn’t like throw away O/L and started doing all the things Olicity. They decided to break this news as they went, like make the transition organic. Not like one day they are saying Laurel is that woman Oliver dropped everything for and the next day, it became all about Felicity. That wasn’t what happened on the show and in the media. When you’re dealing with fandom, you don’t break up abruptly if you want to show that the other option is better and makes sense.
If they literally just had jumped ships during S02, I think the result wouldn’t be as good and it would have been messy. Like, the foundation of Olicity in a romantic perspective came from the first half of S02 and they planned it way before, so obviously all the decisions were made months before they started working on S02 (hence elevator scene and “If you’re not leaving, I’m not leaving”). I think between end of S01 and the beginning of S02 they put up a façade and if you notice, they didn’t talk about how in love they were, how everything would be great before it went to hell, how they’d try to make their connection deepen, etc. They were very vague and repetitive, they didn’t tell exactly what they were thinking to do with O/L and we saw exactly what happened, they just sank it further. I didn’t see a compromise from the writers to do O/L right in S02 and, when I say a compromise, I mean they talking about developing in a positive way that they expect people to love it.
And this got incredibly long, sorry.