This is based on my past experience in purchasing both E and Y Class tickets from SIN.

If you are looking at seats availability when changing date of travel, Y class will give you a better edge compared to E Class on non-fully booked flights. Usually, the E class will be fully booked followed by Y Class.

In terms of flexibility of changes, you have to read the conditions of travel indicated on the website or ticket. For some Y Class tickets, you may be restricted to certain flights or min stay. So it does not really matter if it is E or Y Class ticket. Generally, E Class tickets have slightly more restrictions (such as the flight that you can take), but not always true.

In terms of mileage, both will give you the same mileage points when crediting to your KF account.

Hope this helps.