I have read a few items here but wanted to see if there was any more insight on this issue.
I just picked up a used copy of Adv. Civ but the trade cards are almost too nice and new...they will not mix well with the base set I have which is battle tested and slightly browned, etc.
Is this a huge deal if you arent overly card watching? You could see what good is from what set...what are the other ramifications? Would these all need to be sleeved (except the red back calamities)? What about a sheet to cover the trade cards until its time to actually gather them? You would need to know/see where the calamities are (ie - are they on the top) so not sure thats feasible either but would be easier to implement. From there you would need to keep your cards close and covered so folks cant see what you have.
Could look into a replacement set for the base game but I wanted to maybe try this next week so not sure I would get them in time or if its 'that' big of a deal for a group which doesnt play that often.
I myself had two completely new copies of the games and I still see the difference between the base game and the expansion. It is very slight though so hardly noticeable during play.
I would actually suggest you print out "magic sized" (or other such CCG) cards and then purchase some very cheap common cards of said CCGs and then have card sleeves with a black (or other colour) background so that you can have bigger cards. I played with it once and it was really nice.