Google will update its index gradually, as it crawls your site. There should be very little disruption to search result or traffic if you have the 301 redirects in place.
Do you know whether HTTPS should only be used when the website involves some sort of transaction?
For example if the site was an informational website about collecting coins with no transactions: No need for SSL
If the site is an online shop for buying a car: SSL needed
etc.?
Obviously the Google Ranking Boost would be beneficial but should ALL sites use SSL or only sites involving things that need it?
Any site can use HTTPS, it’s not a problem for a static, non-commercial site to have it.
But only those with transactions or transfer of sensitive data really need it.
It is discussed in more detail here:-
Really, it’s up to you to decide whether you think it’s worth it for sites where you’re not collecting any kind of customer information, whether card transactions or e-mail addresses for a newsletter.
If you want to know what Google says, then I suggest you search their webmaster help.