I don't think there is a satisfactory answer to your question.
The use of "the" with proper nouns is somewhat dependent on arbitrary
choices.
Memorials are not distinct enough, perhaps, to allow the dropping of
"the". Maybe that's because a memorial can take so many different
forms.
the Viet Nam memorial, the Lincoln memorial, the World War II memorial
Airports, on the other hand, and most universities, are treated differently.
O'Hare airport, John F. Kennedy airport, Indiana University, Michigan State University. Except "the" is used with universities when the first words are "University of".
the University of Michigan, the University of Maryland.
He visited the President Kennedy Memorial Hall could probably correctly omit "the", at least to my ear.
Imperial University and National Taiwan University could probably
correctly omit "the", but I think "the" is used in your sample sentence
above for different reasons. Here you want to say that
the X was changed to
the Y.
With "of Taiwan", however, it would definitely be
the Imperial University of Taiwan.
CJ