How to turn off "HEIC" feature in Photos ?

You need to change the format of the photos in the device that takes the photos, i.e. an iPhone or iPad, in the Settings/Camera settings pane:

To convert existing HEIC photos in your library to jpeg just export the file as a JPEG with JPEG Quality at Maximum or High. Then reimport.

Photos doesn't convert jpg files to HEIC.

Only the original file is stored in photos. Any edit previews are normally stored as jpg. It doesn't even offer an export in HEIC format.

Are these photos taken on an iPhone? If so they are more than likely taken in HEIC Format to start with (The default format for iphone pictures)

As Old Toad has already pointed out, the only way to do that is to export as jpg, and re-import. You will have larger files taking up more space, and will lose live photo functionality.

I don't understand why you would want to do this.

This new "feature" (converting my jpeg photos to HEIC) was added to all of my 25,000+

Have you checked the originals on your Mac? If your JPEG photos are all of a sudden HEIC files, your Photos Library has become corrupted. Photos simply does not do this. I have to convert all my older photos (taken before 2012) manually and reimport them to convert them to HEIC.

Select an old photo that used to be a JPEG and look at the filename in the Info - the second line in the Info panel. What is the filename extension? Look also in the second brick, where the file type is shown. Does it show JPEG or HEIC? If the originals are still JPEGs, you just need to revert your photos to the original. Exporting the originals will give you JPEGs anyway.

But if older photos you took as JPEGs and never touched now are shown as HEIC in the Info, some third-party software may have damaged your library. Have you perhaps running some application to remove duplicates? Some "Cleaning" software to save storage? If you let cleaning software modify your Photos Library, you should restore your library from the last backup you made, before the original image files got converted.

If you are having an iPad or iPhone, how are you syncing your iOS devices and the Mac? Are you using iCloud Photos or My Photo Stream or are you downloading the photos manually from the iPhone or iPad?

Do it only for those photos you want to use outside of the library at the time you need to use them.