I forgot to answer your question about how to reflect changes in the file immediately. You can update the Text object whenever you change the file since you change it through an action, preferably in the same event for a more clear process.
On another note, if it fits your project, and as stated by mrcgkh, you could use layouts for different scenes, on your case, if I understand correctly, for the different time periods.
As for JSONs, the basics are using the Object AJAX to request a file (it can be from your project or from the web) and then parse it (put the data) in a JSON Object (a File becomes an Object). This data can be accessed through the expression "AJAX.LastData" inside the condition "On completed" or "On any completed" from the AJAX Object, depending on how many files you're using.
If there are many files, tag them, which then serves to track when they're finished loading and makes it possible to put each JSON file into its correspondent JSON Object easily.
Every change you make should be addressed to the Object and you can see it taking effect immediately.