Eurostar tickets are changeable depending on which
type (or in more plain terms, how much was
paid for the ticket). But, this doesn't preclude you from being all nice and chummy with the Eurostar ticket seller folks and seeing if they'd be kind enough to change the ticket for you free of charge on the day of travel.
When you head upstairs to the
Eurostar check-in level at Gare du Nord, there is a ticketing office where they might be able to do some ticket magic. Butter them up with wonderful stories of Africa. That might help.
I got some good pictures of ticket vending machines in action today. I'll take some time to fiddle with the photos and post them up to the site soon.
p.s. The stories you've heard about African airlines is not uncommon. A friend here in France who works abroad in Africa somewhat often has horror stories of trains and planes that operate on schedules that act as very rough "guidelines". Thus, you're smart to leave breathing room in between connections.