Can I upscale my smart previews to use instead of original images

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Smart previews are 2560 pixels on the long edge. You could Export those photos as 2560-pixel TIFFs back into the catalog stacked with the originals, "baking in" their Develop settings and preserving their other metadata (see the settings below). Then you could run Photo > Enhance > Super Resolution, which will double the long edge to 5120 pixels. Scaling with Super Resolution usually does a good job, though it can't work miracles.

Smart previews are represented as DNGs. If you're experienced with command-line scripting and want to spend many hours, you could avoid the conversion to TIFF and import those DNGs directly into the catalog, preserving the metadata and Develop settings.  Use the Any Filter plugin's Sort command to export all the photos' original paths and the paths of their smart previews to a CSV file. Then write a script to move the DNGs into the folders where the originals were, renaming their base names to the base names of the corresponding originals. Sync those folders with LR to import the DNGs in place. Then use the Syncomatic plugin to copy develop settings and metadata from the originals to the imported DNGs.

I have my smart previews.

By @karenp45708824

Smart Previews appear to be a feature of LRC, so I've moved your post to the Lightroom Classic forum. Please confirm.

What is your OS and your exact version of LRC?

See this help page about working with Smart Previews. 

https://helpx.adobe.com/lightroom-classic/help/lightroom-smart-previews.html

How did you lose your RAW images? 

Jane

This is the Using the Community forum (which is the forum for issues using the forums).
Please tell us what Adobe application you are using so that this can be moved to the proper forum for help.

Scroll down here to see a list of the forums:
https://community.adobe.com/

which app are you using for this?