Small form factor chassis rarely have much leeway for CPU upgrades. Note that all the models are T, or the low power processors, so your choices would be limited because of the heatsink capacity. Also T variants are rather hard to find not already in a system.
The guaranteed list:
- Intel® Core i3-2100T Processor (3.1 GHz, 3MB Cache)
- Intel® Core i5-2500T Processor (2.3GHz, 6MB Cache)
- Intel® Core i5-2390T Processor (2.7GHz, 3MB Cache)
- Intel® Pentium Processor G850 (2.9 GHz, 3M Cache)
- Intel® Pentium Processor G840 (2.8 GHz, 3M Cache)
- Intel® Pentium Processor G620 (2.6 GHz, 3M Cache)
Potentially, H61 chipset can technically run these, but that would be up to Lenovo to make the BIOS capable:
i7-3770T
i5-3570T/3470T
i3-3240T/3250T
All in all not worth it though, at this point you should be looking at getting away from a proprietary small form factor system. Cheapest late model i3 (8th gen and up) or Ryzen processor is going to be a better CPU than anything from the Intel 3rd generation.