Why was it risky? I had two more long distance trains to catch.
So, the first problem is the third train train of the day, the Eurostar, is notoriously difficult to get an interrail reservation on.
Rants on SNCF group's attitude to #Interrail have been had by many in all sorts of places. I'll summarise them as: There is a limited number of seats on the train network, and the operator artificially limits those even further with a maximum number of interrail seats on each train. This sucks.
When I started looking to change my reservations, the first thing I did was try to work out if I could even change my booking. I had the possibility of two later trains, the next one had space in Basic class, and the later train had space in Eurostar Plus, too. And it would be a free change of seat for either one.
So, knowing that, I decided to look at trains from Lyon that would have a more reasonable chance of being doable.
And there I hit my second snag: Train availability. The next possible departures involved a Frecciarossa, which is not available on #Interrail in France, or a Ouigo, which also are not available on #Interrail. The following option turned out to be 2 hours later. And that conditioned me to the last train from Lille.
This is in part because of the check-in procedures. Eurostar themselves tell you to arrive to the station an hour early in order to have sufficient time to complete border formalities. I'm unfamiliar with them on the train, and so I decided to take their advice. So I got an extra hour's wait in Lille, as the timetables didn’t line up well enough for me to have that hour for border formalities.