When ever you change content you should always be careful not to change the key elements that you was previously ranking for. Google has patented technology that monitors changing of key elements such as H1, H2, Meta Descriptions, Titles, so if these elements have been changed its very normal for the page to shift in rankings since Google don't want people modifying pages to improve rankings - after-all why change the page dramatically if its already ranking? So when changing pages you should already keep the key elements while improving the page in other ways such as the content itself by adding to it etc.
Normally its best to change pages over a period of time rather than all at once, so if you've done a lot its likely that Google has picked up on this and is most likely lost your rankings and will improve over time - it's considering those pages as new pages and not existing.
Its like have a page /photography-weddings/ and have the header tags as the same and then changing the headers, titles and so thought to Photography Family Photos and Weddings - such a change will most likely lose the authority it had and has to rework its positioning based on the new page.
Also the fact you mention you've lost 100000 back-links is a major contributor I very much double you actually had those many links from lots of referring domains but rather than a site wide link on a blog roll, footer or something of the sort. These links used to help but nowadays side wide links can actually hurt so its pretty hard to work out if that's the main cause. But I would bet its because the content has been changed too drasticly and effectively Google believes its a new page on the same URL. As a I mentioned early Google has patented technology to detect changes done on a big scale since many people would just keep altering their pages to find out how Google ranks better - and something that Google doesn't want people knowing, or even doing.
Way forward would be to use the key elements from the old page but it may be too late and the best way to get the page back up is TIME. and you should investigate those DROPPING links since that will play a huge part. Dropping links on a huge scale can make your site appear as its been involved in spam, so take a look which links were dropped and so forth.
