In the instance you are making a Halloween DLC style entry of your existing visual novel, you may only use backgrounds and UI elements from the original if they have Spooktober VN Jam themed elements added to them(e.g, Halloween decorations, horror genre props, etc,) if they don't already have them.
You are not permitted to bring over music, sfx, or sprites, or any other unmentioned assets, unless they fall into the realm of publicly available resources rules as listed on the jam page.
For sprites, you're allowed to use the original sprites as a concept for new sprites. In the interest of fairness in the jam, you'll need to recreate them. For purposes of judging, I will say the higher the deviation from the original sprites, while still being recognizable as the characters, the better. New outfits and/or costumes are a good start.
For music, themed remixes of your originals are an acceptable replacement for the original music.
Code, especially for minigames or non-visual novel mechanics, is handled on a case by case basis. If you have code you'd like to bring over, please discuss it with us first. Thus far, we've permitted the use of RPG-like code from a RenPy made original game on the basis that much of its code would be available in another VN-capable engine like RPG Maker out of the box. As a general rule, code that would normally be available out of the box in another engine is most likely okay, but we need to be made aware of it. For judging purposes, I will say entirely new mechanics is favorable to reused ones.
We'll add a link to this question on the page for the rules regarding DLC/Spinoffs.