Constructing user interfaces (UIs) is one of the most resource-intensive tasks in mobile development, often consuming more than half of overall effort. Although declarative frameworks such as Jetpack Compose (Android) and SwiftUI (iOS) have become mainstream, the majority of existing Android apps still rely on legacy XML-based layouts. Migrating these UIs to declarative paradigms is essential for maintainability and cross-platform reuse, but manual migration is costly, error-prone, and difficult to scale. We present GUIMigrator, a semantics-preserving framework that automates the migration of Android XML-based UIs to Jetpack Compose and SwiftUI. We design the Semantic UI Transpiler (SUT), which abstracts layout structures and resource semantics from legacy XML and systematically re-expresses them using the component abstractions and idioms of modern declarative frameworks. This design ensures that migrated UIs preserve both visual fidelity and functional equivalence, while generating idiomatic, compilable code that maintains cross-platform consistency with minimal manual intervention. By separating semantic interpretation from platform-specific realization, GUIMigrator provides a deterministic yet extensible basis for cross-platform modernization, avoiding the unpredictability of purely generative approaches. We evaluate GUIMigrator on 31 open-source applications across ten domains. Results show that GUIMigrator achieves high migration completeness and strong visual similarity (81.9% SSIM on Jetpack Compose and 78.2% on SwiftUI on average), while maintaining substantially higher project-wide semantic coherence (PSC) than modern LLM baselines. In addition, GUIMigrator reduces manual development effort by over 90%.

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