Pokemon-go – switch which egg is incubating

There is infact no way to even reject an egg, at a Pokestop.

Once the egg drops, you can not tell its distance without picking it up.

That said, of the egg drops, and you do not pick it up, the egg will still be added to your inventory. The same happens with any item that drops from a Pokestop.

They expire when used 3 times regardless of the egg. So basically you only have two options:

• Hatch everything with anything:
  • Fast and expensive due to incubators expiring faster.
  • Good for mass hatching to increase chances for 10 km eggs.
  • Just make sure you don't hatch 2 km eggs with a limited incubator when your unlimited one is still available. Use that one first.
    
• Hatch 2 km eggs only with the unlimited incubator:
  • This saves incubator time, but eggs are hatched slower.
  • Better chances to hatch decent Pokémon due to the nature of sitting on more 5 km eggs.

Both ways have their pros and cons.

I don't hatch 2 km eggs with limited incubators (and with the unlimited one only 2 km eggs to make room for new eggs for a 10 km chance), because they're just too rare to waste them on Pidgeys.
If you're aiming for 10 km eggs and you have enough Pokéstops around to refill your egg collection it's better to use your incubators for all kind of eggs, preferably 2 km eggs due to their faster hatch time. This is of course more expensive, but a lot faster as well.