If the poverty guidelines change between the time the petitioner signed the Affidavit of Support and the issuance of an immigrant visa, must the petitioner/sponsor and joint sponsor, if required, submit a new Form I-864?
No, the Form I-864 remains valid indefinitely unless evidence of failure to meet the poverty guidelines in effect on the date of I-864 filing arises. The consular officer will determine whether the income claimed by the sponsor and documented with financial evidence meets the poverty guidelines in effect at the time the I-864 was filed. If the income claimed does not meet the poverty guidelines, then the consular officer may request that the sponsor submit current year income information.
In other words wait till the interview. Otherwise you are probably just creating more headaches for yourself.