Great question. You can change verbs and omit the subject in the second clause, but if you change tenses, you need to repeat the subject.
GOOD: More people like ice cream than hate it.
BAD: I love my current wife more than loved my ex-girlfriend when I was dating her.
GOOD: I love my current wife more than *I* loved my ex-girlfriend when I was dating her.
This is part of the idea of "parallel structure." If you use the same grammar on both sides of a conjunction, you can omit repeated structures, but if you use different grammar, you have to say all the parts of each clause.