Here are the transformations based on the prompts you've provided:

1. We really wish you had kept this plan a secret. (GIVEN)
→ We would sooner **have kept this plan a secret than let it be known.**

2. When the book was published, the writer was greatly criticised. (CAME)
→ Upon **the publication of the book, the writer came in for great criticism.**

3. It is possible that John visited Mary two days ago. (DROPPED)
→ John **might have dropped by to visit Mary two days ago.**

4. More and more people go on a diet now.
→ There **are more and more people going on diets now.**

5. What a pity you missed the party last weekend.
→ I would prefer **that you had not missed the party last weekend.**

6. They redecorated the living room, which was unnecessary. (NEEDN'T)
→ They **needn't have redecorated the living room.**

7. I am sure they will announce the result soon. (MADE)
→ There is every **reason to believe that they will make the announcement soon.**

8. Her friendliness made her the most popular girl in the class.
→ But for **her friendliness, she wouldn't have been the most popular girl in the class.**

Feel free to ask for any further modifications or additional help!