The identity of the new Doctor has always been newsworthy, it's only with the new series that they've been made into true media events (such as the worldwide live broadcast for the announcement of Peter Capaldi in the role).
Now it's seen as being on par with the re-casting of James Bond, both getting bookies declaring odds for various actors.
ADDENDUM - various clippings of articles about later Doctors taking on the role - many more at The O.L.G. Doctor Who Page
The news that Troughton was taking over for Hartnell barely got an inch in the British papers.
Radio Times article with Pertwee
Tom Baker's announcement
Along with the new of the new actor, the requisite "What if they cast a woman?" thinkpieces started popping up as Tom Baker's run was ending. The number of times they were titled "Nurse Who?" is too depressing a number to calculate with any precision. Here is the first "will it be a woman?" article as Baker departs.
ANOTHER "will it be a woman?" article as Davison departs
Readers chime in on casting choices for departing (and misspelled) Davison
Colin Baker's announcement
Colin Baker's departure
Bill Nighy (error)
Bill Nighy was mistakenly announced to be the 9th Doctor.
Teaser for Doctor Who Confidential episode announcing the new Doctor
Extended Interview with Smith from BBC Website
Hurt's retroactive addition to the list of Doctors was the only time there was ever a true surprise about the new Doctors. His regeneration to and from both appeared after this first appearance in Name of the Doctor. They did an amazing job of keeping this secret - review copies of the episode were missing this last four minute sequence.
End segment from worldwide broadcast of announcement
Capaldi's first appearance as The Doctor was in the 50th anniversary episode Day of the Doctor, before his regeneration. As with Hurt, it was kept strictly secret.
Jodie's announcement was made via a special clip premiered on the BBC website.