With the plan to deprecate Grub the Pinephone Pro is switching to systemd-boot, which is now the recommended bootloader. With this update, it's likely that postmarketOS won't be able to boot as Tow-boot (which is the currently recommended bootloader for the Pinephone Pro) can't chainload u-boot.
If you update and can no longer boot into postmarketOS, the easiest way to erase the SPI is to boot Tow-boot from an SD card and select "Erase SPI Flash content".
With this change, postmarketOS will manage u-boot and automatically keep it upgraded on the eMMC or SD card. There is no longer a separate installation/procedure to run postmarketOS on the Pinephone Pro beside flashing a pmOS image.