Elements Spreadsheet is available on Confluence Server, Data Center, and on Confluence Cloud.
Migrating from Server/Data Center to Cloud is fully supported, and does not require any additional step.
When migrating your Confluence spaces to Cloud using the native Atlassian tool, just make sure that your .scc attachments are included in the migration. These attachments are where your Spreadsheet data is saved.
Once your Confluence migration is done, you will instantly be able to resume your work in Elements Spreadsheet, exactly where you left it.
Elements Spreadsheet offers the same features on Server/Data Center than on Confluence Cloud.