Yes, same calendar day, local time, after midnight. And V doesn't need to be available, just at least one seat in Eurotraveller. If you see EuroTraveller for sale on BA.com on the relevant service then barring any weird late recalculations by the underlying algorithm you will be able to do the change online, so long as you have not OLCI. If you think you may be doing this, hold off OLCI since it's actually a doddle to do it on the App. Also don't use e-vouchers or (I think) part pay by Avios, since that takes you out of Self Serve. It can be done by telephone, but the agent may incorrectly start the conversation saying "it will cost you...." - actually it won't.