Since World Collision Radius is a new addition (before world collision was using the center of the particle), setting the default value to 1 can change the behavior of particles in older projects (image attached).

Repro steps:
1. Open Test scene from attached project in 5.4.0b7
2. Select particle system and observe how particles go between the cubes.
3. Open Test scene in 5.4.0b13
4. Select particle system and observe how particles no longer go between the cubes (because default collision radius is 1).

Notes:
- Technically a regression, but also a new feature. Used to work in the mid-betas (circa b7).
- Might not be as problematic and is easy to fix (setting radius to
- Versions listed are a bit of a guestimate, not 100% sure when the change got merged :)