The pawprint symbol does not represent a cost, mana, counters, or any kind of persistent resource. You can't "save up" pawprint symbols from one Season spell to use on a future one, mostly because there isn't anything concrete to save up. They're just pawprints.
If a Season is copied, the effect that creates the copy will usually allow you to choose new targets, but you can't choose new modes.
Choices made for the first and second modes of Season of Gathering aren't made until the spell is resolving.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Irina Nordsol
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If all targets for the chosen modes become illegal before the Season resolves, the spell won't resolve and none of its effects will happen. If at least one target is still legal, the spell will resolve but will have no effect on any illegal targets.
You don't have to choose modes that add up to exactly five pawprints. For example, you could choose the two pawprint mode twice and leave it at that. You could even choose no modes at all. You probably shouldn't, but you could.
No matter which combination of modes you choose, you always follow the instructions of a Season in the order they are written. If the same mode is chosen more than once, you choose their relative order as you cast the spell.
You choose the modes as you cast the spell. Once modes are chosen, they can't be changed.
No player can cast spells or activate abilities in between the modes of a resolving spell.
If a mode requires a target, you can select that mode only if there's a legal target available. Ignore the targeting requirements for modes you don't choose. Each time you select that mode, you can choose a different target, or you can choose the same target.
