New features in this release
From Audacity Development Manual

This page is an overview of prominent new functionality that has been introduced in Audacity 3.1.0
Version 3.1.0 is ...
- Details of all the major changes since 3.0.3 can be found in Audacity Release Notes 3.1.0.
Contents
Clip-handles
Peter 36Jul21: ToDo-1 provide section on Clip-handles (formerly known as "affordances").
TBP
Bug fixes
P1 Bugs
None
P2 Bugs
None
P3 Bugs
None
P4 Bugs
None
P5 Bugs
None
Updating from Audacity 2.x to Audacity 3.x
With the release of Audacity 3.0.0 the project file structure of Audacity was changed significantly with the entire project contained in a single unified database file.
If you have already updated to any 3.x version you should already be aware of this.
But if you are updating from any earlier 2.x Audacity version please read Updating from Audacity 2.x to Audacity 3.x.
Older Projects:
Audacity 3.x can usually open AUP files created in older 2.x.x versions of Audacity , but saving the project in 3.x will prevent it opening again in Audacity versions prior to 3.0.0.
- Note that this will leave behind the old AUP file and its associated _data folder which you will probably want to delete to recover disk space.
- Audacity version 3.x should be able to open projects from Audacity 1.1 onwards - but not from 1.0
Links
> Audacity Release Notes 3.x,x - detailed release notes for this release of Audacity