upgrade the 2960S-48TS-L to same IOS (15.0.2a.EX5) as the 2960X-48TS.
sauce: click here
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.
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.
I did that and the switch could not boot, stopped halfway while executing. I could recover from ROMMON. On checking I found 15.0.2a.EX5 is only for 2960X-48TS-L models. The nearest available software for 2960S-48TS-L is 15.0.2.EX5. There is no 2a available on Cisco software downloads.
Do you think this will work when stacked?
I thought so. But while trying to download the 15.0.2.Ex5 for 2960X-48-TS-L there are warnings of bugs on that software and that is the reason I am trying to ascertain if anyone has tried this. I cannot afford to bring down both switches using that buggy software.
Try 15.0.2.EX5 on the 2960S-48TS-L. If that doesn't work you might have to rethink stacking the switches and just run a trunk between the two switches and run them as separate switches,
Upgraded both switches to 15.2(2)E3 and both switches stack fine without any mismatches. You need to download software separately for 2960S and 2960X and upgrade the switches. This is the stable version as informed by TAC and worked fine.