Why Is Excel Suddenly Messing Up My CSV Files? Used to Work Fine.

I've used Excel for years to edit CSV files of data for a client's ecommerce store. Only recently, however, Excel is converting all my HTML special characters, significantly messing up the data. I've goggled and googled but cannot figure out what settings I used to have. I did not change any settings prior to Excel no longer handling my CSV data well; must have been part of an update?

Guidance much appreciated. Under a very pressing deadline and wasting so much time having to take the extra step of opening the edited CSV, search for all the odd characters, do find-and-changes to get them back into HTML characters before finally uploading the data back into the ecommerce store.

FYI, save the original CSV into XLS format for editing, as I've done ever since I lost a day's work on an extensive spreadsheet that went wonky because I'd continued editing in CSV format. Then when done with editing, I export to CSV -- only recently I no longer get any options on what type of CSV to save-as? Used to get to choose the ...-8.

Thanks!