1. Y is the generic code for Economy as well as being a specific economy fare code. If you have a 'proper' Y fare i.e. fully flexible then you should be able to change BUT it will depend on availability and if there is a difference in the fare you will have to pay that. If you check in 'manage my bookings' it should tell you there what the actual fare code is rather than the generic cabin code.

2. VS only release 60% of seats for pre-request in advance and the rest will generally be opened when OLCI opens (but worth checking a few hours before hand)

If you do want to change flights then I'd be on the phone to VS now checking if I could change flights and if so how much it would be