Unless sponsor policies state otherwise, supplemental funding will be considered new funding. Any application for new uncommitted funding, such as a supplement, should incorporate the new rates.
Exception: The National Science Foundation has a policy of funding supplemental support using the negotiated indirect cost rate(s) approved at the time of the initial award, so supplemental proposals to NSF should use the original agreement's rates. See NSF PAPPG Chapter X.D.1.d.