Just to report back, calling in to change the destination to a different country worked, and I was not charged an extra phone fee. However, I was charged the $300 rather than the $199 change fee. I mentioned that the change fee displayed during the booking process was $199 but the agent said her system shows the $300 fee and she suggested that it might be because I changed the destination. I didn't see anything in the "PENALTIES" vs. "VOLUNTARY CHANGES" sections of the fare rules I included in my previous post to suggest that it was indeed the change in destination that triggered the higher fee, but I didn't press it. I might follow up with an email to customer service but overall I'm happy that I was able to change the destination which the website hadn't allowed me to do. This wasn't a trip for which I thought when I booked it that there was a significant chance of having to change it or else I wouldn't have selected the economy light fare in the first place, but such is life.

Thanks again for all the feedback and advice here, greatly appreciated. This was my first booking with KL in a long time so I'm not as up-to-speed on rules and practices as with some other airlines.