CHICAGO--(BUSINESS WIRE)--Fitch Ratings has upgraded CEMEX, S.A.B. de C.V.'s "CEMEX" Issuer Default Rating IDR to 'BB-'from 'B+'. The upgrade is supported by the strengthening of CEMEX's capital structure during the past two years due to the conversion of about $820 million of convertible bonds to equity plus the refinancing of around $8.6 billion of debt that has lowered the company's annual interest expense by approximately $200 million per year. Also factored into the upgrade is the improvem