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Is there a way to toggle back to pixelated (original) mode with no high res rendering?

Something like the images shown here:
Post edited August 22, 2019 by RodCastler
Not something "like" that, exactly that. Those are the original graphics.

I own only the originals, so I wouldn't know for sure how to do it, but the 1 & 2 25th anniversary editions say that you can in their store page:

Optional retro settings: play with original graphics, original music and even the original controls (mouse pointer)
So, if it the game did not come with any instructions on how to do it, and it is not clear from in-game menu options or something similar, you should contact GOG support and ask them how to do it.

--- EDIT ---

Ah, I see they did the old "but you can have the old version too" move.

From a review in the store page:

The only saving grace is the original game is available as a bonus in the gog goodies (it's "Legacy version").
You need to download a separate installer from the "backup & goodies" of the game
Post edited August 22, 2019 by Links