The biggest risk when translating from http to https -- this is a temporary loss of the site from the index (old has already fallen and new ones have not ranked as high), and, as a consequence, loss of traffic for this period.
First, if possible, for the old site better to either skip or minimize it: for example, translate to https only sensitive sections (e.g., sections of the basket and the payment for the online stores). At the moment, just the fact of working over https gives significant advantages in the search.
If the switch to https all the same it is necessary, here is the algorithm how to minimize losses in this move:
1. Raise the website to https. At this point, the website needs to function correctly both on http and https.
2. Add both sites (http and https) in "Yandex.Webmaster" beta.webmaster.yandex.ru.
3. In robots.txt both write Host: https://site.ru/
(substitute your domain, don't forget to consider looks as the primary mirror-with www or without).
4. For an http site at "Yandex.The webmaster is" check application to transfer the site to https.
5. Wait. When Yandex and Google completely thrown out of the index the https site, only the absolute then put a 301 redirect from http to https.