Persistence of .pkgsave file content

Until a few hours ago, I assumed that, in a .pkgsave situation:

- if a .pkgsave file exists, its content will be lost
   (replaced with newer content).

For example, if user grahamperrin exists in
/etc/group but _not_ in
/etc/group.pkgsave

then a pkgbase upgrade would result in
/etc/group.pkgsave _including_ grahamperrin.

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 From test results a few hours ago, it seems that:

- my assumption above was false

- it is advisable to delete archaic .pkgsave files before a pkgbase upgrade.

Can someone confirm?

Thanks