once switching over to https killed a lot of my search engine positions and it took 1 year, if not 2 to recover, however i probably did something wrong on my end and do not know what excatly...on the other side leaving other sites untouched (
http://) did cause no harm to their rankings so i decided to keep my old sites mostly untouched as long as possible
however i do propagate new sites usually with https:// because its just so easy nowadays, and converting later to https:// can be cumbersome