I am improving the SEO for out website. According to the best practices, I have changed more than 3 thousands of our content titles to become SEO friendly:

1. Shortened them to under 70 characters (they where more than 130 chars.)
2. Changes the keywords according to the google trends.
3. and so on...

I am pretty sure that the SEO changes that I made to my pages were really good, but I have seen that Google removed some of my pages from the first page of the search results. I dont' know where are they now! I could not find them in first 20 pages of Google! They where in the first page, but now they are not even though they are more SEO friendly! 

I know that it is indexed in google. It shows the result with a `site:` search.

How long does Google need after performing title changes before my site is back again in the results? Does Google normally demote a site when it detects a lot of changes in the website contents?