If the text file is being transformed into an HTML page, then it shouldn't be much of a stretch to parse that same information into an RSS template as well. Just publish the HTML and RSS files at the same time.
If you want to keep older entries in the feed simply don't remove them. Just keep appending new entries to the feed. You'd want to be careful that the feed doesn't become too large though if you accumulate too many entries.
You can set a cutoff limit so that the RSS feed doesn't have more than say the most recent 25 updates. It really depends on how often you update and how many older entries would be useful to your feed subscribers. A newspaper would have many stories a day and perhaps would want to keep a much larger feed active.