Hi Mu,
Each day's Canvas Data files will contain the current version of the entities in the Canvas application. Canvas Data helps you find deleted entities, which no longer appear in the Canvas application, but for active, available, and unpublished entities, Canvas Data only has the current version.
That said, you can easily access the five most recent daily dumps of Canvas Data files in the Admin panel's Canvas Data Portal link. If the discussion reply you're looking for was revised within the last five (or so) days, the oldest of the five versions you see there probably has the original version of the discussion reply.
If the discussion reply you're looking for was revised more than five days ago, you may be able find the original discussion reply in the dump created the day the reply was posted (although it is possible that the student posted the reply, then edited it, all between creations of daily data dumps). You can install the Canvas Data CLI tool, which will enable you to list and download the last 50 daily data dumps. More information is at How to Use the Canvas Data CLI Tool.
I hope this helps.
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After I posted the answer above, I decided to verify that I could download the oldest data dump listed by the Canvas Data CLI tool. It turned out that for the 20 most recent dumps, I could see all of the tables, but for the 30 older dumps, I could only see the requests table's files. So, you may see something similar, meaning that you cannot actually go back 50 days. Sorry for posting originally without testing.
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