To clarify, somewhat - if you epublish, you've published that work. Period. Virtually no traditional publisher will even look at that work afterward except as a reprint (at a substantially reduced royalty and advance rate (if any at all)).
Once an ebook is published somewhere like on Amazon or B&N or Smashwords, it's there forever until you take it down and it counts as having been published because the internet's backup servers have it stored on those servers forevermore. There're also all the folks willing (and able) to burn disk images from web files they have their armies of little ebots up online searching.
Basically, once it's up on the web, it's there in some fashion forever until something catastrophic takes the whole system down (and fries all the backups - something exceedingly more difficult to do every day).