Reading the docs I found this paragraph that shows in more detail how plots work:
A plot is like a sub-chart, containing one or more series and drawn as
a particular chart type, like column or line. This distinction is
needed for charts that combine more than one type, like a line chart
appearing on top of a column chart. A chart like this would have two
plot objects, one for the series appearing as columns and the other
for the lines. Most charts only have a single plot and python-pptx
doesn’t yet support creating multi-plot charts, but you can access
multiple plots on a chart that already has them.
From your question I assume that you want to delete the plot to replace it with a new one. I suggest that you delete the series inside the existing plot and add to it the new series you need.