Happy to report a success story (my first), got CSU applied to an upcoming reservation at IC Lisbon, booked a Queen Classic on a 5000/night bonus point rate. Called the diamond support line, got a great agent that applied the CSU and got the last King Junior Suite Park view confirmed. Pretty happy with the value for money given the rates have gone up for that period quite a lot since I booked!

Took about 15 minutes in all, had to hold a few minutes while he confirmed the upgrade.