Many magic items (including some weapon and armor runes) have effects with save DCs. For example, a weapon with the frost property rune gets the following critical effect:
On a critical hit, the target is also slowed 1 until the end of your next turn unless it succeeds at a DC 24 Fortitude save.
The frost rune (and some other magic items) has a greater version with a higher DC, and it is possible for a higher-level character to upgrade the frost rune to its greater version, but it is effectively a different item.
Because the save DC is constant, it does not scale against higher level targets, and the item becomes less effective over the course of campaign. Aside from simply replacing the item with a better version, are there ways to increase the save DC on an existing rune or other item?