After an extensive discussion with Doug Gill (solo events, technical
manager) at SCCA regarding the legality of the Praxis system in SCCA solo events, I can report that within the current rules structure, Praxis legality is as follows:

Stock Classes (S) - not legal *
Street Prepared classes (SP) - not legal **
Prepared classes (P) - legal
Modified classes (M) - legal
Street Touring classes (ST) - not legal **
Street Modified (SM) - legal

* S classes do not allow spring replacement.

** SP and ST classes require that the spring be replaced by a spring of the same type (coil with coil, torsion-bar with torsion-bar, etc.). Even though the air-spring does not alter the location or direction that the spring force is applied, it would be considered different and, therefore, not legal.

By analogy, it is interesting that other, very common and popular aftermarket products face this same quandary. Upgraded engine computers face the same classification status as Praxis.