Recreate old aliases in bulk to reduce their file size, keeping their name and creation date
On macOS 10.14 Mojave, inspecting any individual legacy alias with Get Info (⌘I) causes it to be regenerated with a smaller size.

To use Get Info to regenerate aliases in bulk, first show a list of the aliases to update in Finder in List View. To find all your aliases, press ⌥⌘Space to open a new search in Finder, click ‘+’ to add an criterion, and set the criterion to “Kind” is “Other”, typing in “Alias”.

In this list of aliases in Finder, select the top result, then press ⌥⌘I to Show Inspector. The selected alias will be regenerated – you will see its size go down. Now hold down the ↓ arrow key to select each alias one at a time. Each of them will be regenerated, getting an updated Date Modified but keeping its Date Created.

You have to inspect each alias one at a time because inspecting multiple aliases will only update the first one.