It's always interesting to me that folks refer to any change as an "upgrade". Some are, but most are just changes for fit or appearance, not necessarily "better" equipment.

To OP: if you want to change (note I did not say upgrade) your stem just for a different look, go for it if you can afford it. But unless you can find a carbon stem that weighs a meaningful amount less than 100 grams, it won't be an upgrade at all. (a syntace aluminum 26.0 stem weighs about 100 grams and isn't seriously expensive - probably about the same as a Bontrager stem that weighs 30% more)