I am on award travel, too -- all business. There was no availability online for business class in Saver on the website. I called, and they fixed it.

I am also on award nights at the Conrad, with pre-devaluation miles, and also cannot change the dates. Exactly the same issue.

Just call Singapore Airlines and ask, point out that it's an involuntary change, and it causes a misconnection to the seaplane to access the hotel, and explain that the hotel reservation is non-changeable (and I would add "not refundable", as it basically isn't refundable in a like-like manner due to devaluation) and that the previous day works better for you. As long as you booked through Krisflyer, I bet they will help you. Be polite, but insistent -- THEIR changes screwed things up for you. THEY should fix it.

If you booked through another Star Alliance partner, it may be tougher, though.