Many years ago, my first PCompatible (after 10+ years with a TRS80 Model 1) was a 386SX (Dak if it matters). Several years later, I wanted an upgrade but couldn't afford a whole new computer. So I got a Cyrix "486" kit (Intel had real 486s, but nothing that worked on a 16-bit bus like the 386SX had) that alleged to upgrade the 386SX in place. It clipped on - push it down over the 386 and it slid over its pins, apparently put the 386 into permanent reset in some way, and took over (386SX was soldered in). The Cyrix also came with a standard Cyrix '87 coprocessor, which the 386SX had an open socket for. IIRC the upgrade didn't improve much, but it didn't cost much, and didn't make anything worse. The '87 did seem to help a little with spreadsheets. Performance was probably more limited by RAM (I filled the empty RAM sockets a short time later, which probably made most of the observable difference in performance) and hard disk performance anyway, give what was available at the time. At least, it kept it working well enough to last a couple more years, until switching to Windows 98 required a new Celeron (P5) machine.