Updating your motherboard's BIOS — its UEFI firmware — sounds like it should change everything about the firmware, identifiers included. In practice, a BIOS update normally leaves your firmware-level identifiers exactly where they were.
What a BIOS update changes
A BIOS or UEFI update replaces the firmware code: the program that initialises your hardware and starts the boot process. Manufacturers release updates to fix bugs, improve hardware compatibility, support new components, and patch security issues. The update rewrites that code. What it generally does not do is wipe the data values the firmware stores alongside its code.
Why the firmware identifiers usually survive
The SMBIOS UUID and the BIOS serial number are data — specific values written to identify the board, typically set when the motherboard was manufactured. A firmware update is designed to update the firmware program while preserving this kind of board-specific data. That is deliberate: these values are meant to be stable for the life of the board, and a routine update is built to carry them across. So after a normal BIOS update, the SMBIOS UUID and BIOS serial are normally the same as before.
The exceptions worth knowing
"Normally" is doing real work in that sentence. Behaviour can vary between manufacturers and update tools, and edge cases exist — a board that shipped with a blank or placeholder identifier, or an unusual update process, can behave less predictably. The honest statement is that a standard BIOS update is expected to preserve these identifiers, not that it is guaranteed in every possible case. If the exact value matters to you, the way to know is to check it before and after.
What is definitely unaffected
The identifiers that have nothing to do with firmware are untouched regardless. Updating the BIOS does not change the Windows machine GUID, which is in the registry. It does not change disk volume serials. It does not change MAC addresses. A BIOS update is a firmware operation; the software and disk identifiers are a separate layer entirely.
The takeaway
Updating your BIOS rewrites the firmware code but normally preserves the firmware-stored identifiers — the SMBIOS UUID and BIOS serial are expected to carry across a routine update. Behaviour can vary at the edges, so check before and after if the value matters. Software-level identifiers like the machine GUID are a different layer and are not affected at all.