Season 15 ends on the 11th of March 1978. 25 weeks later on the 2nd of September, Season 16 commences with "The Ribos Operation" and Doctor Who's first Season arc - "The Key to Time".
Actually this isn't such a radical departure for the show, as there's a certain continuity between one story and the next as each story ends with the Doctor and Romana having gained one more piece of an ongoing puzzle. Which makes it quite similar to Seasons 1 to 3...except of course that each story does not lead directly into the next, with a recap of the previous episode.
Does the general public like the new format and the new companion, posh Time Totty Romana? Well there is a drop in ratings compared to Season 15, but whether this is due to ITV competition or simply the 4th Doctor beginning to wear out his welcome, there's no real way of knowing. If you want to compare Seasons, then 16 starts with 8 million viewers whereas 15 started with about 7. On the other hand, the last 7 weeks of Season 15 saw viewing peaks of 10 to 11 million, so go figure. They say the Devil is in the Details, and unfortunately we don't have all the details.
It's a fact though that no less than 6 million are tuning in each week (with the exception of part 4 of "The Armageddon Factor" where viewers drop to 5.6), and most weeks see at least 2 million more. Not only that but:
Season 16 averages out at 8.5 million across its 26 weeks, which fandom today has a tendency to see as part of the show's decline since Season 14. However this still puts it on a par with the 3rd Doctor's highest-rated Seasons 10 and 11, and arguably makes it part of Doctor Who's continuing ratings success since 1970. It is still being watched by more people than Hartnell's 1st and last seasons, and the entirety of Patrick Troughton's tenure.