Not necessarily - could retain backward compat by publishing both gzip and zstd variants and having downloaders with newer npm’s prefer to download zstd. Over time, you could require packages only upload zstd going forward and either generate zstd versions of the backlog of unmaintained packages or at least those that see some amount of traffic over some time period if you’re willing to drop very old packages. The ability to install arbitrary versions of packages probably means you’re probably better off reprocessing the backlog although that may cost more than doing nothing.

The package lock checksum is probably a more solvable issue with some coordination.

The benefit of doing this though is less immediate - it will take a few years to show payoff and these kinds of payoffs are not typically made by the kind of committee decisions process described (for better or worse).