I finally have decided to upgrade to Trueos from PCBSD 10.3. Following the instructions of the handbook, I have a ffew problems in the process:
1.- The USB stick at boot time complains with wrong partition table!. After pressing "enter" the installation goes on smoothly.
2.- When the installation finishes after the reboot, the new system is not in the boot menu.
3.- I log in my old pcbsd, and find with beadm that actually there is a boot image called 12.0 current. I reinstall GRUB.
4.- Now the new system appears in the boot menu, but if I choose to boot it, I get an error zpool.cache not found.
5.- This problem seems to be similar to https://discourse.trueos.org/t/moving-from-grub-to-bsd-loader/1104 but the solution proposed there is not valid form me because I have MBR HDD (lecacy BIOS).
Now the system seems to be there but I am unable to finish the upgrade. Are there instructions available for this upgrade for legacy BIOS with an MBR HDD.