Hiya,
We had one trip via Lille and I must say I don't want to ever do the same thing if I can avoid it. The change itself wasn't bad, but it was running around to find the right platform, nothing was well explained and there was noone to help. Luckily, my DH speaks French and managed to find the way to get to the platform. The train from Lille isn't an Eurostar one so it was a little different but not so bad. Sorry I may sound negative but really didn't like it (maybe I was too emotional as we travelled with a 4 year old and 3 month old in December)
As for the return journey, I'm sure you can swap places with some people esp if you go on the direct train. Everyone using Eurostar direct to DLRP is lovely and very helpful.
Enjoy your trip.