Which function would you use to change the case of character strings?

Using the UPCASE function is appropriate when you want to change the case of character strings to all uppercase letters. This function effectively converts each character in the specified string to its uppercase equivalent, making it ideal for situations where uniformity in letter casing is required, such as standardizing data entries or preparing data for analysis.

The other functions listed serve different purposes: the LOWCASE function is used specifically to convert strings to all lowercase letters, the SUBSTR function extracts a specific substring from a character string based on position, and the TRIM function removes leading or trailing blanks from a string. These different functionalities highlight why the UPCASE function is specifically the correct choice when the goal is to change character strings to uppercase.