Just read a post on a bird watching forum, a chap had updated the firmware on his 150-600 and it had stopped working with the latest Canon converter.
He contacted Sigma who looked into it and confirmed it was the latest firmware that had done it,then the normal statement that they could not guarantee Compatability with third party converters.
This is where in my mind the good service comes in,they said if he wanted to return his lens to them they would roll the firmware back to the previous version, i know of at least one major camera maker that would not have wanted to help.
He contacted Sigma who looked into it and confirmed it was the latest firmware that had done it,then the normal statement that they could not guarantee Compatability with third party converters.
This is where in my mind the good service comes in,they said if he wanted to return his lens to them they would roll the firmware back to the previous version, i know of at least one major camera maker that would not have wanted to help.