It might not have been economically viable. The pace of obsolescence at that point in time was so fast that it might have been cheaper, over the course of two or three years, to just buy a whole new system than to retrofit a Pentium to a 386 mainboard.

Consider what else happened in the 386 through Pentium transition:

* The PCI bus became standard; your peripheral cards all needed to change.

* SDRAM became standard; the memory bus became significantly wider / faster, and you bottlenecked on RAM if you used older mainboards.

* IDE, while it was already used for hard drives, was now used for your CD drive too - you probably wanted a CD drive and not a second floppy drive (different controller) in your machine, which was harder if you only had a primary IDE bus.

I recall upgrading my 286 to a 468. Correction, it was one of those Cyrix 486 clones on a 368 motherboard[1].

I bought the motherboard, CPU and RAM. Half the cost was the 8MB of RAM. CPU was the next most expensive item.

Worked fairly well, but it was speed limited in certain cases by the 386 motherboard/memory combo. Also not all applications/games would run since they didn't properly detect it as a proper 486.

[1]: https://ancientelectronics.wordpress.com/tag/486-in-386-moth...

They cost almost as much as the real thing and were barely faster than your starting position in most cases.