Using Wasm64 and enhanced orthogonal persistence in Motoko, is an opt-in feature during the beta testing and initial roll-out phase. Users can select this by the compiler option --enhanced-orthogonal-persistence. The migrations happens automatically. Please note that reverse migration from 64-bit to 32-bit is not supported. You may find some useful information in the following post: Beta Testing Motoko’s Enhanced Orthogonal Persistence (EOP)
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