reservation change to earlier train

Hi I have a train booked and a seat reservation paid using Eurail web site already leaving Paris Disney to Lille Europe for a 9:01 train how would I go about changing the reservation to an earlier 7:50am train. I have already added the journey to my trip in the Eurail app. 
this journey isn’t until Thursday 3rd of August. 
I am just a little worried about the connection time from Lille Europe to London. Arrive Lille Europe at 10:17am departure at 11:35am might be tight clearing boarder inspection.