In reply to jon:
I have turned up a day and even 2 days early and been charged between nothing and €10 for the alteration.
What I tend to do is book longer than I need the pay a small supplement when I decide it's time to go home.
Be aware though.
Last year I was booked for Saturday lunchtime but my estimation was I was going to get there on Friday at 9pm, with the last train being 10pm I decided to ring them and make sure I could change my booking.
The 9pm booking was going to cost €70!! But the 10pm was only an extra €7 and because as standard you have 1 hour either way of your booking time I still got on the 9pm train anyway.