Transition matrices are an important determinant for risk management and VaR calculations in credit portfolios. It is well known that rating migration behavior is not constant through time. It shows cyclical behavior and signiflcant changes over the years. We investigate the efiect of changes in migration matrices on credit portfolio risk in terms of Expected Loss and Value-at-Risk flgures for exemplary loan portfolios. The estimates are based on historical transition matrices for difierent time horizons and a continuous-time simulation procedure. We further determine confldence sets for the probability of default (PD) in difierent rating classes by a bootstrapping methodology. Our flndings are substantial changes in VaR as well as for the width of estimated PD confldence intervals.