The transfer process is pretty easy. You first apply online through Target's site and notify your HRETL.From there they'll do a phone and/or in-person interview. If your going to IB or OB, you'll have to do a physical abilities test at the DC. Then your in. A word of caution though, it can be difficult to get into some DCs that don't hire very often. However, right around now-Sept is usually the best time to start applying, as we're usually looking for bodies to prep for Q4.
And no, you shouldn't have to repeat the drug test. A caveat though, whatever your off-the-clock activities may be, if you hit something with a vehicle or cause 500$+ in damages, they'll make you piss in a cup, so keep that in mind.