I found the official poster for the Playful Kiss Youtube Channel, and the tagline gives me renewed hope about the content of the upcoming Youtube episodes of PK [Edit, this has been extended to 9 episodes in total now].
The tagline reads:
Sneaking a peek into the newly-wed life of Ha Ni and Seung Jo.
This really must mean that the 9 Youtube episodes have to comprise new footage. Putting two and two together, the writer likely wrote 20 episodes worth of material. When it was decreed that PK would be a 16 episode drama, the best-of-the-unfilmed material is aggregated to create 9 episodes on Youtube, each 10 minutes long.
This means we get 90 minutes total of new PK to look forward to after the drama wraps this week.
Link to original news article: