This is apparently an unpopular view even among grammarians, but I don't think it's ever okay to use 'different than' in any form. The words just don't go together. Taking an example that was presented as a correct usage, I'll try to illustrate:
He is different now than he was in high school.
He is different now than {he was} in high school.
The word 'different' is nothing but the adjectival form of the verb 'differ.' When a thing is different from another thing, the first thing differs from the second. One should be able to switch between the forms without offending the ear. But you would never say:
He differs now than he was in high school.
He differs now than {he was} in high school.
or even
He differs now than the man he was in high school.
The problem with saying that 'different than' is appropriate when "
t introduces a clause (whether the verb is stated or is implied)," is precisely that it encourages us to forget what 'different' is supposed to be used for. It's a word that compares two like objects or concepts that have some difference between them. When you use it to 'introduce a clause' you're no longer comparing an object to an object, or a concept to a concept. You are, almost every time, comparing a thing to a manner of being. A thing cannot be different from a manner of being. They have nothing to do with one another. They can't be compared.
He (thing) is different now than [he was in high school] (manner of being).
He is a person. The way he was in high school is not. The only way to correct this, I think, is to compare like to like.
He is different now from the man he was high school.
The way he is now is different from the way he was in high school.
I think 'different than' never works.