Some new and changed features of Revit 2021 result in certain behaviors for upgraded models. Review this information before upgrading your models.
When you upgrade cloud models to Revit 2021, Owned Elements are relinquished to preserve data scalability. Note that Owned Worksets are retained as usual.
The realistic view engine has been replaced. Upgraded views are much faster and look much better. However, maintaining the display of materials and lighting to provide the same image on the screen in Revit 2021 compared to Revit 2020 is not guaranteed. You may need to modify view settings to get a similar display for the view and it may never be exactly the same as Revit 2020 views set to Realistic display.