Not sure about one with a cast that changes with the seasons, but Ninja Senshi Tobikage (aka, Ninja Robots) and Ronin Warriors come to mind. The former looks like it was broadcast via CN in a number of countries, but the latter feels like a better fit with your description.
Thanks for the reply! Looks like Ninja Robots did air in my country, but the plot doesn't feel like it and the art feels a little different than what I remember, so I'm not sure. Ronin Warriors seems more likely, but after a little researching it looks like it has never aired in my country...
Of course, I read, "when I was a child," and immediately think of Steel Jeeg (1975-76), so some timeframe might be helpful for context. ;)
2006-2014. The art in my memory feels like it's from that time. (But at this point my memory is so bleak that I can't trust myself to remember things correctly lol.)