If I have a lower-level Aid cast on me, am reduced to less than 5 HP, and then have a higher-level Aid cast on me, what happens?

Question

For reference: Does casting Aid stack? (No), What happens to the current hit points when Aid ends? (You lose the current hit points that you gained, effectively taking 5 damage).

My question is:
If I have a 2nd-level Aid spell that increases my current and maximum hit points by 5, then drop to 4 HP, and then have a 3rd-level Aid spell cast on me, what happens?

I see a few possibilities:

1. I return the 5 HP I gained from my 2nd-level Aid, drop to 0 HP (and start dying), and then the 3rd-level Aid spell brings me back to 10 HP with no delay.
2. My current Aid spell is simply superseded by the 3rd-level Aid spell, so the circumstances simply change to being granted 5 current HP to 10 current HP. Since I'm still due 5 HP, my current HP goes up by that value (now at 9) and I and never touched the Dying condition.
3. Another possibility that you can explain.

Which of the above is true?

Answer 1

When the duration multiple spells of the same name overlap, you use the effect of the most potent. From the rules on combining magical effects (PHB, p. 205):

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

So, the first casting gets suppressed and replaced by the second casting. Never does the first spell stop, unless or until the spell ends - but its effect is replaced with the more powerful spell effect. Which would be your option "#2".