How to continue incremental learning when a categorical variable has been assigned additional category labels?
Looks like a special case for incremental learning / life-long learning. There are many papers in this line of work:

https://arxiv.org/abs/1611.07725

https://openreview.net/forum?id=BkloRs0qK7

Most try to preserve the model parameters in a way that reduces the loss of previous knowledge ("catastrophic forgetting"), but some also try to find (or keep) some samples and preserve the original model's response on those samples.