The datastore cannot reflect the changed values because it simply reads the data from the file into a table.
If you want to store the changed values, you want to write the modified table back out.
Example:
while hasdata(ds)
[T, info] = read(ds);
T.DateValue(T.DateValue < 1997) = 0;
filename = ['newfolder/newfile_', num2str(info.Offset), '.txt'];
writetable(T, filename);
end
newds = datastore('newfolder/*.txt');
While this may not be the most efficient, it certainly will work.
Hope that helps.
