The word convirtieron is the preterite form of convertir in the third person plural.
See the full convertir conjugation

convertir

Esos civiles se convirtieron en el ejército de la República.
These civilians were converted into the Army of the Republic.
Y tu esposa e hijos se convirtieron en cenizas.
And your wife and children have been turned to ashes.
Terminó con una inundación y sus habitantes se convirtieron en peces.
It ended with a flood and its inhabitants were converted into fish.
Tal vez convirtieron la fábrica en una instalación de arnéses.
Maybe they turned the factory into a harness facility.
También los acreedores privados convirtieron sus tipos de interés en préstamos.
Also private creditors converted their interest rates into loans.
La mayoría de los hombres prominentes del lugar se convirtieron.
Most of the prominent men in the community were converted.
Ahora recuerdo cómo esas horas se convirtieron en días.
Now I remember how all those hours... turned into days.
Los políticos convirtieron esto en una crisis internacional.
The politicians have turned this into an international crisis.
Los vi tomarlo, pero se convirtieron de todas maneras.
I saw them take it, but they turned anyway.
¿Estás diciendo que ellos se convirtieron en espíritus malvados?
So are you saying that they've become evil spirits?
Y entonces cuando nos fuimos, Frank y Dennis se convirtieron el colega-colega.
And then when we leave, Frank and Dennis became all buddy-buddy.
Al final, miren en lo que se convirtieron.
In the end, look at what you have become.
Sí, lo convirtieron en un centro de rehabilitación de drogas para mujeres.
Yeah, they turned it into a drug rehab center for women.
Los vi tomarlo, pero se convirtieron de todas maneras.
I saw them take him, but they became anyway.
He encontrado las pinturas, se convirtieron en dinero en efectivo.
I found the paintings, you turned them into cash.
Quizá convirtieron la fábrica en un centro para los captadores.
Maybe they turned the factory into a harness facility.
Parece que se convirtieron en un tazón de petunias.
Apparently, they've turned into a bowl of petunias.
Ustedes tomaron la tierra y la convirtieron en propiedades.
You took the land and you turned it into property.
¿Por qué convirtieron este cuarto en un calabozo?
Why do they make this room such a dungeon?
Estos arrestos preventivos se convirtieron en un sistema.
These preventive arrests were built up into a system.
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