There is a standard behavior of the platform to archive activities if certain conditions are met
- Events due more than 365 days ago
- Closed tasks due more than 365 days ago
- Closed tasks created more than 365 days ago, if they have no due date
As stated here
the 365 can be increased up to 2555 days. Fine. Now here
I found that increasing this limit won't unarchieve activities which are already archived even if they are below the increased threshold. Also it's not possible to write this flag explicitly.
What is the best way to unarchieve those tasks according to the new threshold?