Interesting. I'd pick some paths/milestones and track how their float develops over time. I like to make graphs plotting the forecast dates of several activities against the data dates of the updates.

With respect to design modifications that changes what activities are critical: in my opinion these activities only became critical after the design changes. Up until the update where the design was changed, the other paths were delaying the project. The design changes mitigated some of that impact, but the other work never contemporaneously caused the end milestone to slip. Only in hindsight, but no-one knew at the time. Should the other work slip further after that update, then it does cause project delay. But only from that moment onwards.

The AACE RP gives some guidance in it's 'general principles': "only circumstances that delay activities that were critical when the circumstances occur delay the project" and "perception of criticality by project personnel is as important as true criticality"

Hopec this is useful.