Currently the protocol does not support canisters being migrated across subnets. There is an MVP version of subnet splitting where subnet A can be split into two subnets A’ and B, each hosting a subset of the canisters that were hosted on A before the split. This MVP version comes with quite some downtime for the subnet to be split; so I think it would only be used as a measure of last resort in case a subnet fills up. I’m not aware of any work planned to further extend the MVP to support more efficient splits and/or migrations of canisters in the near future.