I have posted elsewhere on the issue you raise. To save me re-typing again please see link below.
But quick summary:
I think it depends on whether Zwift is recording your best Watts
Or is recoding your best Watts/KG.
First one - weight changes have no bearing (provided no PB watts are achieved) on CE category.
Second one - potentially a huge bearing on your CE category.
And of course my answer has absolutely nothing to do with the question as to whether your weight change has any other beneficial gains or affects other Zwift riders.