Short answer
HubSpot's KB: migrated articles get temporary URLs ending in /migration for review. Upload images to the files tool first. After you publish, the original becomes read-only for 30 days. A banner means the new knowledge base isn't published yet.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Publish soon after migrating so customers aren't stuck between versions.
3. Related
Multiple knowledge bases and styling.