V4 and V5 are both very simple to upgrade with the K6-III, since they both have switch SW4 for setting Vcore from 2.0 to 2.8v. Just set Vcore to 2.0 v, plug in the K6-III and go. If you want a larger multiplier than 3x, you have to play with R130, R327 and R328 (details in other threads).
V3 requires some soldering (details in other threads). Basically, you have to enable split-plane functionality (separate Vcc and Vcore) and you need to set up the voltage reference divider to give 2.0v as the reference.
For the split-plane, desolder R60 and R61 and install it at R184 and R185.
For the voltage reference, as suggested by Turbo3, use R342=25k and R343=100k for the required ratio. (R342 has a 10k that needs to be removed. Simply moving the 10k to R343 and adding a 2.5k at R342 will not work, since it loads the reference and drops it below 2.5v).
Then, set up the voltage-series feedback divider for the op-amp U16. This consists of R202 and R203. The details are in the other thread, but for Vcore=2.0v, just remove R202 and you're set.
Before firing it up, remove the CPU and plug in the AC adapter. Measure the voltage at the source of Q16 (this is also conveniently available at one of the pads of R60 and R61). It should be the Vcore you have chosen (say 2.0 v). If it is not in the range 1.9 to 2.1 v, do not plug in the CPU until you have rectified the error.
If Vcore is within spec, you are ready to install the CPU and go. If you want to alter the multiplier from the default 3x, the procedure is the same as that for V4/V5 (R130, R327 and R328).
I haven't looked at V1/V2 yet, but I expect that they will be similar to V3.