Cambodian tourist visa stamp to new passport

To get to the point, before reentering Cambodia from Thailand, I applied for a new passport in Bangkok. After a fortnight I was notified that my new passport is ready for pickup so I left Cambodia in order to collect it from Bangkok and get back to Cambodia again.

At the borders (Poipet) I presented both passports with the Thai exit stamp in the old one. The Thai officer asked for some additional docs given by the embassy in similar circumstances and he also mentioned that the embassy officials should have canceled my old passport when I received the new one, which unfortunately wasn't the case. To cut things short, they let me pass by stamping my old passport so I entered Cambodia with the same passport which expires in October.

I contacted my embassy in Bangkok and they advised me to go in the local immigration office in Cambodia to pass the Cambodian entry stamp in my new passport.

The next day  I went to the local police station and they told me to go to an airport (Phnom Penh or Siem Reap) to get this over with. I am sure there must be some other department to deal with similar immigration situations but the officers didn't seem to know anything about it.
   

So as it seems the only choice I have is to visit one of the two airports' immigration office.
Does anyone know if there is another gov office or department elsewhere?

Thanks in advance for any info.