When we switch over to using the Library Card Platform as the default application process for all partners we will also want to support and add all the existing distributed accounts. This data will be necessary if we want to be able to process renewals through the platform, and for phase two/three we will need to know who has access to what.
This isn't strictly necessary until Phase 2/3 - we can always email users directly about renewals, but I think renewals are something users will expect to be able to ask for through the platform.
There are a number of directly necessary features that will need to be included in the platform for this to happen, which are listed as subtasks of this one, and normal or higher priority subtasks of T147434 are - for the most part, and only in my opinion - the less direct improvements that should be made before going ahead with this.