S&P Global Ratings proposed changes this week to its criteria for rating non-agency MBS. The changes would provide higher ratings to certain tranches of non-agency MBS backed by new prime mortgages while prompting lower ratings for new issuance backed by seasoned mortgages. The rating service is considering a number of adjustments to foreclosure assumptions, loss severity projections and changes relating to the evaluation of qualitative factors. “The proposed revisions to our methodologies and assumptions are...