It is great to see that when the Curl 6.0.5 IDE is saving/closing a 3.0.x project that it no longer saves the CPRJ file with metadata about styles. The effect of this was to render that CPRJ file invalid for a developer still using a Curl 3.0.x IDE (project would not load). The work-around was to manually edit the CPRJ to remove the default styles info (only 3 lines near the top of the CPRJ file) or to try not to commit it to source version-control (easier said than done when working under pressure.)
Thanks folks. Much appreciated!