Actually, I would change in Cologne irrespective of which train you can catch to depart DUS airport, as Cologne Hbf is a much nicer place to change, and I really see no need to stay on the RE between Cologne and Aachen, if you could also take the ICE (at the same price). Additonally, it reduces the risk of losing the connection, as your layover in Cologne will be 29 minutes as compared to the 14 minutes when changing in Aachen.

However, there's a catch involved, should you choose a non-flexible ticket ("Sparpreis"). If the connection that will be displayed during the booking process requires you to change in Aachen, you basically cannot change to the ICE in Cologne. In this case, you should add a 20 minute layover im Cologne in the appropriate search field. Even if you only want to change in Aachen (for whatever reason), this will be much safer for two reasons:

If you can only catch the later train (RE5) to Cologne, your ticket will stay valid (you can take any kind of local train before changing to a long distance train) as you can just jump into the ICE at Cologne (or do the same thing if the RE1 is delayed and you might not catch the ICE in Aachen, this is actually not too unlikely, I have often seen the ICE overtake the delayed RE in Eschweiler, only 3 stops from Aachen Hbf), and the ticket would still be valid for an RE journey between Cologne and Aachen if you choose not to change in Cologne. However, if you buy a ticket where the ICE part only starts in Aachen, you cannot take the ICE from Cologne (at least not without buying another ticket at ca. 22 EUR p.p. for the short 30 minute ride).

Buying a flexible ticket, however, would eliminate all those issues, as you can basically use any train between the Airport and Brussels (including an IC/ICE to Cologne) except Thalys.