normaly you can just install on top of the old vs. currently the trial is the classic game with foundations expansion and the SI graphics and GUI.
for gold, you will just need the original CD key that comes with that box set. yes do retain your trail CD key, that is the CD key your account is bound to. if you ever need help, CS, TS, what ever, it will all be tied to that CD key.
once you have gold installed, i would patch it, then install ToA behind it.
that is what i did. i had my original classic CD from years ago (like the day it came out), then upgraded like you to gold/toa combo.
for me it was an easy install, but on here there have been a few ppl to have issues with it. i would not stress to much over it. it rather simple. put in the CDs in the proper order and follow the on screen instructions and you are set.
i even changed the default path for the install as i do not install anything on my C:/