Happy new year!
When operated on 5V, the ST link chips are also active on the K8 board. Vcc is still 3.3V, therefore the RPi signal levels are still 3.3V. It's not a necessary modification, but it can help in certain scenarios, there is no real drawback to doing this.
The power consumption increase is not significant, because Vcc has not changed, only the ST link is powered up. This has a low contribution to the overall power consumption, hence this modification was built into the update. The hack can be done in two steps.
- Remove SB4 from the top of the board. This will disconnect the 3.3V supply.
- Create a connection with a piece of wire between the 5V RPi output and the 5V ST input (pin 19 on the schematic).