Over the pass few years much has changed for data structures within Appian, specifically with record types and custom data types (CDT). There does not seem to be any official documentation on how to convert an application designed using CDTs to one using records. While CDTs have not been entirely fazed out but rather relegated to support with plugins and smart services, it is often more performant and (based on Appian documentation) preferred to build using record types.
My questions are:
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CDTs are not being deprecated... And records can be backed by CDTs still, so I guessing you are referring to synced records.For existing applications, there is no need to convert - but consider building new when your business case/requirement is to leverage new functionality. It is not technical debt, but instead opportunity to build to enhancements. You can have a CDT backed record and and synced record against the same entity. But do this when you want to take advantage of new features (driven by business needs) to do things like use self-service analytics. If you do have tech debt of a query using a CDT backed record that maybe exceeds the query time limit, maybe then consider switching that to a synced record since that will "fix that tech issue" -- but otherwise, do this based on enhancements requests and needs.