Say bank A buys a credit portfolio "B" (e.g. corporate loans or retail mortgage, ...) from bank B.
Bank A fulfills the the requirements of CRR (capital requirement regulation) for its existing portfolio and applies the IRB (internal ratings based) approach. Thus (among other things) it has a risk model that estimates the probability of default and that can assign customers to one of $N$ risk classes.
How can A prove to the regulator (in this case ECB) that it can integrate the new portfolio "B" into its existing one and treat it in IRB? What analysis should be done to show that the risk model in bank A fits for "B" too?
This is not a pure brainstorming question but rather a best pratice/regulatory one. I would be happy about any references where this was done (and some report was published) or any opinion how this could be done taking into account the regulatory framework and quantitative analyses.