More information, please.
Is the rule a part of a paragraph style? If so, then yes, just edit the style. If they are applied locally, not as part of a style, and are really paragraph rules, you may be in for a lot more work. Paragraph rules are "adornments" and are not accessible to Find/Change, so there's no easy way to step through them. You could define a new paragraph style, though, with the rule included, then apply it to any paragraph that has a rule. That would be faster than setting the weight for each one.
If the rule is an actual drawn line, then you can use Find/Change in Object mode to locate and change them.
This is a bit off the wall - but if you tinker with this I think it can work.
Be sure to do this on a copy of the file
You can select all the text and use File>Export and choose InDesign Tagged Text
You can search for the Rule Above in the txt file and replace the value there.
Best to set up the rule above weight you want to use at the start of the document so that's accessbile for you when open the TXT file.
Then do a search for pRuleAboveStroke:
And replace that Above Stroke value with the value from the start of the document
You can then import that Txt file back to inDesign.
(I did test this with bringing the file into InDesign as TEXT and GREP seaching for
And replacing with
To make all strokes .5)
Not sure if that's a good idea though - seemed to work ok?