This new revision of the hardware adds level shifters to the address lines, fully protecting the RP2040 from 5V signals. It also adds an external reset pin to the board which can be hooked up to a reset line on the target hardware allowing it to be reset after the ROM image has been changed. The picorom tools reset command controls the reset line.
There are now two versions of the firmware. PicoROM.uf2 is intended for the new 1.5 hardware revision. PicoROM-1.2.uf2 for the older 1.2 hardware revision. Both versions will work with both hardware revisions, however the reset pins and LEDs will not function correctly.