Hi there, 3 year JIRA/Confluence user, but 1st time community poster!
We are currently under going a project to migrate our (currently 'live') on-prem install of Confluence to the Atlassian cloud version. We have been working through the documentation (pre-checks, installing the Confluence Migration tool, ect), but we have only one concern before going forward:
If we initiate the migration process, will all spaces and articles be removed from the current on-prem install of Confluence as it is migrated to the cloud?
Apologies if the above question is silly(!), but we just want to make sure so we can plan our approach going forward.
Many thanks 
Craig.