Flight change

Hi guys!

I really hope someone can help me.
I've booked a flight for December 20th including a bag of 20 kgs.
However, I'm considering flying on the 22nd.
It is probably cheaper to buy a new flight instead of changing the old one.
But is there any possibility to cancel the bag in my old reservation? Or can I "move" it to my new flight?
Moreover, high season starts exactly between these dates, on the 21st to be exact.
If I do decide to change my flight, instead of simply buying a new one,
will my changing fee be the one for the low season or high season then?
And does this also mean that if I change the flight, I have to pay extra for the bag,
as it costs 10 euros more in high season?
Also, I will then have to pay the difference in flight costs. Is this simply the difference in the stated price on their website when booking or the price you pay in the end, so including administration fee etc.?

I hope to hear from you soon!