Does transcoding reduce quality?

It depends on what you transcode to and at what settings.

First generation Apple ProRes 422 will perfectly preserve the quality (be it good or bad) of the original file.  That is, the resulting file will be no better nor worse.

If you're producing video for broadcast, you'll want to go with Apple ProRes 422 (HQ) or Apple ProRes 422.  If you're making videos for social media, you'll want to consider going with Apple ProRes 422 (LT) to save on storage space requirements.

An example of transcoding that will cause a loss in quality would be transcoding to H264 at a low bit rate.  The compression generation loss would be immediately apparent in the resulting file.

Anyone interested in Apple ProRes and quality preservation should read the following white paper:

https://www.apple.com/final-cut-pro/docs/Apple_ProRes_White_Paper.pdf

-Warren

In your specific case, I'd expect there to be no visual difference.

Yes every transcoding costs quality. Some codec are visual lossless.

But why do you need prores?

Cannot your machine handle xavcs?

Might want to look into proxies and the Cineform codec.

PROXY