It is impossible to view oldest changes if those exceed the 500 - 1000 limit.
- Make rcdir=newer and rcdir=older work on recentchanges pages
- Make it work as a simpler initial implementation only available as a url parameter
- Make it work exactly the same way as the API(see API:RecentChanges or&list=recentchanges&rcdir=newer&rclimit=500)
A full interface is not part of this suggestion.
When combined with recentchanges from parameter (e.g. Special:RecentChanges&from=20181207124145) this would effectively be a poor man's version of navigation for older changes (pagination). For example, it would allow users to do something like Special:RecentChanges&from=20181207124145&dir=newer, and would work pretty much like https://www.mediawiki.org/w/api.php?action=query&format=json&list=recentchanges&rcstart=2018-12-01T12%3A46%3A39.000Z&rcdir=newer&rclimit=500.