I would like to check if both 3750 Switches are stacked together, if I upgrade one of the 3750 switch, will the other switch IOS being applied as well?
Hi, when you upgrade the stack master switch the IOS changes should be done on the member switches. Switches without the same IOS versions and stack protocol version number are not compatible with the stack.
Catalyst 3750 Series Switches in the stack might not boot with the new image after a software upgrade. This issue might be caused because you used archive download-sw /leave-old-sw in the download option.
The /leave-old-sw option keeps the old software version after a download. When you enter reload, only the stack master is reloaded. This fails because the switch as a stack expects all models in the stack to have the same version of the image. As a result, the stack master switch is placed in a disable state, and another member switch is elected as master.
Read document below for more help