How can I change all links in the store to be https if the site accessed via https?

[Website Rob]

I did not want to switch the entire site to SSL.
All I was looking for was for the http://domain/ to have all http:// links, but if arrived at the site via https://domain/ for all links to follow the same and be https:// links. Is that possible? If they were relative links, they would inherit the protocol, would they not?

Is that possible? Yes

Is it worth the effort? No

Bite the bullet and go SSL for all pages. I will disagree that SSL everywhere is a Google thing, although it may look like it at the moment. Once servers make the upgrade to using HTTP 2 (which is much better than what is used now), SSL for all pages becomes mandatory anyway regardless of whether the site is eCommerce or not.