Hi. Congratulations on your purchase and welcome to the forum.
It would be best to change it before storing it. Oil picks up contaminants, some of which can be corrosive, and I would imagine even more so on a new engine. Best not to let it sit there munching on your engine. Is your mower going to blow up if you leave it over the winter with the break-in oil in it? No. Would it be best to get it out of there and store it with fresh oil? Yes.
So that's the FX691V engine, right? I have that engine on my 52 inch Bob-Cat approaching 270 hours and it has been flawless.