Is it possible to return to the old version of my contacts if the treatment done by Dropcontact does not suit me?

If you are not satisfied with the results provided by Dropcontact, no problem! It is quite possible to return to the initial version of your contacts:

Restore an old version of your contacts on Google contacts

To determine the time at which the changes started, just look at the time at which we sent you the first email whose subject is: "Your file being processed".