The phrase "up on which" can be replaced with several synonyms to enhance the clarity and flow of a sentence. Some suitable options include "on which," "over which," "above which," "upon which," and "regarding which." These phrases function similarly, conveying that whatever follows is in reference to a specific topic or subject. "On which" is the simplest variation and could be used in everyday conversation, while "above which" or "over which" may be better suited for more formal or technical writing. Ultimately, choosing the appropriate synonym depends on the context and tone of the sentence.