Hope some of you already have some hands-on experience with that - if you're adding a new switch to the stack and you get IOS version mismatch, can you still copy the bin image from the master to the new member over the stack cables?
Major Stack Protocol Version Number Incompatibility Among Stack-Capable Switches
Switches with different major Cisco IOS software versions usually have different stack protocol versions. Switches with different major version numbers are incompatible and cannot exist in the same switch stack.
Minor Stack Protocol Version Number Incompatibility Among Stack-Capable Switches
Switches with the same major version number but with a different minor version number are considered partially compatible. When connected to a switch stack, a partially compatible switch enters version-mismatch (VM) mode and cannot join the stack as a fully functioning member. The software detects the mismatched software and tries to upgrade (or downgrade) the switch in VM mode with the switch stack image or with a tar file image from the switch stack flash memory. The software uses the automatic upgrade (auto-upgrade) and the automatic advise (auto-advise) features.
The port LEDs on switches in version-mismatch mode will also remain off. Pressing the Mode button does not change the LED mode.
So, if the version are "close", the auto-update feature should take over. Otherwise, a manual upgrade will be required. (copy the firmware from the existing switch via usb, scp, etc. to the new switch.) I know this can be a pain when they're both in your hands, trying to walk someone through it remotely is tough.
I was finally able to test it in the production environment at my company and can confirm that no USB was necessary, despite the "Major Stack Protocol Version Number Incompatibility" - it was still possible to access the flash of the mismatched member from the master of the stack.
As for the auto-update feature, I must say that we only use bin images at our company and so that feature has never ever worked with our 2960x stacks, unlike what Cisco brags about in its documentation. Each time the IOS had to be upgraded manually, regardless of how close or distant to one another the IOS versions were.