Does the lack of wa change the sentence?

So I saw this sentence: きのう初めておすしをたべました。 And wondered if the は is just being left out. Does the sentence change any if は is added in? Like so: きのうは初めておすしをたべました。

Simply, before experts explain.

きのう初めておすしをたべました
is very ordinary.
きのうは初めておすしをたべました
is emphasized about きのう.

I'm exaggerating a bit, it's like this.
"What did you do yesterday?"
"Yesterday? Speaking of yesterday, I ate oSushi for the first time."

Umm..., excuse my poor English.(x_x;