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Mostly it is shown. A couple of them weren't shown. We never saw the Sixth Doctor turn into the Seventh, because that series had ended with bad blood and the actor did not wish to do the regeneration scene. We didn't see the Eighth Doctor turn into the Ninth, because the Eighth Doctor only got the TV movie in the 90s with the failed attempt to reboot the series as a Brit-American co-production, after a hiatus of almost 10 years.
The Eighth Doctor featured in comics and also audios during the years between the tv movie and the 2005 reboot, but I think they wanted to start fresh by that point and didn't want to begin the new show by harking back to the last time it had been on the air.
Actually, that first series was almost devoid of references to past Doctors. An odd line or two here and there, a few old monsters. Also the website shuffled the past Doctors out the way so you really had to go looking for them if you wanted to know about the series history. Once the show had found its feet a bit they started throwing in little references and bringing in the odd actor/character from the old show (like Sara Jane) and gave the past Doctors more prominence on the website.
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