I've filed a utility patent application, with a number of claims that increased the original filing fee.
Is it possible to change that application, to reduce the number of claims, resulting in the reduction of the original filing fee?
If you did not pay yet (no refund...): yes - a readable blog post which says:
"In a US national stage application filed with excess or multiple dependent claims, the USPTO will issue a Notice of Insufficiency which will provide the applicant with an opportunity to amend the claims to reduce or eliminate excess claims and multiple dependent claims".
Also here - the MPEP, which says (see section III (V)):
"In situations in which a payment submitted for the fees due on filing in a nonprovisional application filed under 35 U.S.C. 111(a) is insufficient and the applicant has not specified the fees to which the payment is to be applied, the Office will apply the payment in the following order until the payment is expended:...(8) the excess claims fee (37 CFR 1.16(h), (i), and (j))". Which means that some claims will not be paid for and will therefore not be searched/examined (you do not get to choose).