For migration from Jira Data Center to Jira Cloud you can find the tools here to perform your migration.
Use the tools with guidance from the SodiusWillert support team. In the package, you will find HTML help pages to understand the usage flow. Please submit a request for an activation key to use these tools.
oslc-connect-jira-cloud-tools_1.1.0-WIP-v20261020-1626.zip