I can confirm that in ~10 years being AMB every single time received an upgrade (for >70 stays at ~30 different IC hotels).
I typically make a 'strategic booking':
Highest normal room category - upgrade to at least a Junior Suite
Junior Suite - upgrade to Full Suite
From time to time, I was upgraded to special Suites that are normally excluded for upgrades (Ambassador, Royal, President, ... Suites etc.). In my experience, I rarely see the upgrade shortly after booking, in maybe 20-30% of stays 1-2 days before arrival, and hence most of the time at check-in.