I am considering going through our list of items and editing their default values.

The question I have is this: If I edit the default values, is this going to change the DKP points spent by people who purchased these items previously and were entered at the default cost at that time?

Example. Uber_sword_01 dropped 2 months ago. Its default at that time was 30pts

Now however that sword is only worth 5 pts.

I don't want people to suddenly find they got 25DKP back if I change the default value to 5.

Only if you want it to. When changing the value of the item, it should prompt you on whether or not you want it to make the change retroactively.

This popup only comes on the single edit page though. The quick edit is just an implied "No".