It's not been confirmed but it should go like this:
Final 3 - judges scores don't count, public vote alone
Final 5 - both judges scores and public vote combined, but no dance-off (bottom two are both eliminated)
Other weeks - current format maintained i.e. judges scores and public vote combined plus the dance-off.
I doubt there will be a dance-off in the semi-final. It's never a good idea to eliminate the celebrity who is top of the public vote, which was probably the case in series 5 (Gethin). Since then the producers scrapped the idea after the Tom Chambers fiasco, as there was no semi-final dance-off in Series 7.
This series is actually unprecedented, since in previous series where there was a dance-off there were only 3 couples in the semi-final. Those three series (series 5, 6 and 7) were meant to have four couples in the semi-final and three in the final; but in all of those years, a couple withdrew from the show (Kelly and Brendan; John and Kristina; Jade and Ian), resulting in a single elimination leading to a two-couple final, or no elimination leading to a three-couple final.
The current five-couple semi-final format is a contingency plan, in that the show can revert to a single elimination if anyone withdraws.