Using several IDX clusters from one SHC is doable and supported configuration. You need two separate CM one for each cluster.
Migrating indices and buckets from one cluster to another can be do, but there is something what need to fulfill to get it work. Much easier is if you could start data collection from scratch in the new cluster and let those old indexes frozen after retention period. If this is not an option I think that it’s better to ask help e.g. splunk PS or some local splunk partner.