You'll have to read the rules for the ticket you have. Each booking class is different, and they vary sometimes from route to route even for the same booking class.
I think all W tickets I've seen do not allow changes whatsoever. I've only bought those for domestic routes though. V tickets I think may allow a change, but for a fee. I've bought lots and lots of international Q tickets, and never had a problem changing them without any fee, but once had one where the TA told me it couldn't be changed without a fee, but TG allowed the change for free. I've never seen anything where changes to an earlier flight the same day are treated any different than a change to another day. A change is a change and it's either allowed or it's not, and with a fee or not, regardless if it's an hour early or a week early.