Firstly, there are plenty of games that use a mix of pixel art and high res art. Even Celeste, which uses the biggest pixels I've seen this side of a ZX81 in-game, uses high res and even 3D for the character dialogues, menu systems and intro.
So if you want to use your original art, there's no reason why not. Try to keep a consistent feel across the two styles though. You'll have to take off your artist's hat and put on your art director's hat. This is not the easiest thing to do.
If you want to convert to pixel art, you could try to minimise banding. For example, the chest-plate has an even grad across it and this makes for obvious banding in pixel art. You could try bunching up the bands over smaller distances, reducing the colour count and making it more of a glint across the top with shadow nearer the bottom.
Similarly with the hair. I don't think you need the long mid-tone tufts down the left so much when you have the lovely thin bright wiggly sheen nearer the top.
But this is all my personal preference. YMMV.
Something to remember. "Pixel art" isn't really a style, it's a medium. There are as many styles of pixel art as there are artists. Moving to another medium doesn't NECESSARILY mean you're shifting style, but it's more likely I guess?