I think that's a good question, but a tough one to answer. I imagine it'll be different for everyone. Generally, I think it's going to be better to "under scope" and expand on what you have, than "over scope" and feel like you're under time pressures the whole time.
For me, knowing the state of games I make in that short of a time, I'd be thinking that being done (something playable and fun) in 8 hours sounds great! Now I have 40 hours to really have fun with it and experiment. There's a ton of different things you could do.
I would try to take a step back and look at the game for areas I can improve based upon the jam's rating criteria. Maybe adding a bit of story elements could really increase player engagement. Or now suddenly there's enough time to implement a goose as a final boss from all these spaceship parts, or do something else wacky with the assets there didn't seem to be time for originally. Or add extra polish. Which could range from particle effects and screen shaking to hover states for menu buttons, or settings for adjusting sounds and such.
Just two cents from someone who still considers themselves a beginner :) Best of luck!