How to replace raw files with jpegs but keep them in collections?

follow up question: in the export menu, i can choose in addition to lossy compression a resize to a smaller image size (for example 3000pixel instead of 6000pixel long side). sadly i cant find this option in the "convert to dng" menu. is there a way to resize maintaining dng in collections? thanks in advance!

you can find jaos comprehensive answer further up in the thread!

Ahh different can if worms

 Two related concepts

1 A photos crop proportions, as in 8 by 10 as opposed to 4 by 3. In Lightroom these are proportions not actual dimensions, accomplished in develop.

Now editing the crop will not remove the photo from any collection (except a smart collection based on crop, who knows)

If you want multiple crop versions in a collection, then create virtual copy's and crop them independently and add the to appropriate collections.

2. A photos actual size as in pixels, inches, or megabytes etc. For example 8" by 10" or say long size 1024 pixels . This is only accomplished during an export.

If you want multiple photo's resized to particular dimensions, then export a copy from the original, using appropriate values and renamed slightly as you see fit, then add to appropriate collection

What you are seeing is happening as designed. When you create a collection the original file remains in the folder where it is located in your HDD, the collection does not contain copies of the original files it just data stored in the catalog file. So you can have the original referenced in multiple collections, however, if you delete the original it will be removed from all the collections.

When you export a jpeg file from your raw files a new file is created so you will now have two original files one a raw and one jpeg. Create your collections from the jpeg files.  See the screen capture, in the Library module the original files are in the Folder section and then there is a Collection section. You should notice that they are displayed differently.

Ok, re-reading your post I see I clearly do not understand the problem. Above may be of absolutly no use.

Inquirys

1. It is your JPEG exports that are disappearing from collections?

2. What collections?

3. Normal user created collections (not user created smart collections) or smart collections?

4. When did you place the  exported JPEG photos into collections?

5. Do the JPEG exports still exist? Can you edit them??

6. Any error messages?

First up, I will try very hard to ignore the decision to trash the originals.

Moving on, collections

Photos in collections are virtual, Collections do not actually store photos. 

Photos can be members of multiple collections 

Photos can be added automatically to smart collections

Exporting a photo, say exporting a RAW photo you edited, does not add it to a collection. Unless it meats some  criteria for a smart collection.

The photos, both original (until you deleated them) and the exports exist not in the catalog but in one or more folders on one or more hard drives, be they internal or external (photos can also be on a NAS, not the catalog, just photos). 

If you had photos in a collection and they disappeared when you deleted your RAW photos, then those were probably the RAW photos you deleted. Hmm, but why would RAW photos you have no plan to keep be in any user created collection)

Now one major issue one major problem. We're your RAW and exported JPEG stacked together when you deleated the RAW?

And one oh by the way. All Photographs is actually a collection, (an exception to photos not being place in non smart collections, just for the picker members)