Coming off this morning's story named Google: It's Wrong & Bad To Tell Your Clients Not To Go HTTPS, Gary Illyes from Google added that when you go from HTTP to HTTPS, you should not see any "long term negative impact when switching protocols." And by long term, Gary said a couple weeks after the migration.
Here is the tweet where it he says no long term impact when migrating:
@DaveNaylor can you guys give me examples (sites)? There shouldn't be *any* long term negative impact when switching protocols.— Gary Illyes (@methode) August 18, 2015
When he was asked how long is long term, Gary said two, maybe three weeks:
@DaveNaylor one week is understandable, two weeks, with a stretch. For any longer I'd raise an eyebrow— Gary Illyes (@methode) August 18, 2015
So if you have migrated to HTTPS and you saw a seriously bad long term impact, it was either because you had technical issues with your migration, you have some other issue with Google around the same time and/or Google is wrong.
In all my tests, I did not see long term Google issues with migration from HTTP to HTTPS.
Forum discussion at Twitter.