What determines whether a partial TeX upgrade can be safely resumed?

In contemporary TeX Live maintenance cycles, a distribution upgrade that terminates mid-installation may leave fmt files in an inconsistent state, as fmtutil depends on complete binary writes and index updates. When the upgrade process is interrupted, the filename database generated by texhash might not reflect newly installed classes or packages, potentially causing silent typesetting failures. The core uncertainty involves whether a partial upgrade can be safely resumed by re-running the package manager or if the resulting binaries require full replacement to maintain typesetting reliability. Documented behavior shows that fmt file sets generated from incomplete installations can produce missing-format errors without immediate crash, making diagnosis difficult.

Can fmtutil reliably reconstruct missing formats from a partially upgraded tree? Does tlmgr's --force flag preserve custom class files when resuming an interrupted upgrade? Should users treat any upgrade interruption as requiring a complete binary reinstallation?