The org I inherited was created back in 2009. For Households a custom record type named "Household/Individual" was created. NPSP added "Household" record type. Any advantages changing existing Household/Individual record types to the NPSP record type?

If you are wondering why you should move from a currently customized account type to NPSP's standard model, my view is for the sake of sustainability and future alignment with NPSP features. If you perceive your customization and NPSP design as two paths forking from the time you installed NPSP, then continuing your design may make it harder for you to utilize all the features available within NPSP.