Legacy application integration guide
You don't need to rewrite an existing app to add PTERI — it integrates through the same API.
An older application doesn't need a full rewrite to adopt PTERI — the integration path is the same REST API and SDKs described in integrating PTERI into your application, added incrementally alongside whatever authentication your app already has.
A common approach: keep your existing session/login system in place initially, and introduce PTERI-backed authentication or step-up authorization for the specific actions or flows that need it most, before expanding further.
Being upfront about what's not published yet: there's no dedicated legacy-migration tool or pre-built adapter for common older auth systems today — if your architecture has constraints that make this tricky, open a ticket (Technical / API category) and describe your setup; that's exactly the kind of case that should inform what we build next.