What do you mean by "migration"? As with any other similar solution you typically don't migrate data already ingested/indexed/saved/whatever in the original solution. You just spin up a new Splunk environment and onboard sources you had sending to the old SIEM (often if possible you try to have a transition period when sources send to both old solution and Splunk). Any "logic" like assets databases, alert rules, reports and so on have to be manually created to give you desired business outcome. It's best if you engage your local friendly Splunk Partner in such project.
