Yes. According to ASTM A563, the specification covering carbon and alloy steel nuts, the supplier may supply any higher strength nut at anytime, but the supplier must give notice to the purchaser that the substitution is being made. The supplier cannot supply a nut that is prohibited by the inquiry and resulting purchase order.
“1.2 The requirements for any grade of nut may, at the supplier’s option, and with notice to the purchaser, be fulfilled by furnishing nuts of one of the stronger grades specified herein unless such substitution is barred in the inquiry and purchase order.” -ASTM A563 Specification
The A563 specification contains several other nut grades that could potentially be used in place of an A563 Grade A hex nut. Refer to our nut compatibility chart for other suitable nuts that are an acceptable substitution for an A563 Grade A hex nut.