If cgc determines the final value belongs in a higher tier, they could charge you more. If it should've been in a lower tier you won't get a refund.
If u you look at the fees, it's a few -$80 diff unless you're talking 'walkthru'. You're probably gaining a lot more value than if it stayed in the original tier.
However, I've seen posters say they received books in grades, which should've put them in upper tiers, but never were asked for more.
I've also seen posters state they were asked for an additional charge. I don't know the rhyme or reason as to why or why not.