So I need to get from Dusseldorf airport to Lille-Europe at the end of May ... I haven't used the rail system in Germany for almost 15 years and confess that I feel out of my comfort zone here in understanding the rules and regulations ... I understand that I have to take the exact ICE/TGV train I have ticket for, but that for regional train connections I have some flexibility. What I am not clear on is whether this also applies if the first train is a regional train.

What I would like to do is DUS - Koeln (around 10:00), spend a couple of hours there, then pick up an ICE train from Koeln at 13:43. So basically I would like to book:

dep. 12:50 DUS->Koln[RE 28517] arr. 13.29,
dep. 13:43 Koln->Bruxelles-Midi[ICE210] arr. 15:35,
dep. 16:17 Bruxelles-Midi->Lille-Europe[TGV9836] arr. 16:53,

but then instead take the first (regional) train available to Koeln after arriving in DUS at 09:30 (rather than waiting for the 12:50 train).

Does anyone know if that is that allowed?