Technically, the Wiki could just "Redistribute" or "Republish" everything on here under 4.0 and there is nothing to stop us. That's how CC-BY licenses work. As long as we make sure to properly credit them, upgrading the license is fine.
I don't see any compatibility problems here either — 4.0 can use anything before that, and if anyone out there can't be bothered to upgrade their own license, that's fundamentally on them.
However, I don't think it's particularly nice to just do a blanket roll-out. As a compromise, we can work with a hidden tag, i.e. _cc-by-sa-3.0 to mark such articles explicitly wished by the author to remain accessible under 3.0 license.
SCP-UA is using a license notice in the sidebar including the nifty Creative Commons license badges. Nothing stops us from wrapping those into Iftags and having it also display in the sidebar for all 3.0 articles.
Come to think of it, WD has not default 4.0 license to display in the footer. So we probably have to use the custom HTML option for that… well, at least we can also use images then.
Nest question is: what should we do with Items where the author is no longer available to tell us what happens with their stuff? Thinking of it like organ donations, we need to have some sort of default answer. I'd go with opting-in (for licensing, that is; you all should be organ donors) because doubt that somebody who hasn't show themselves in the past years is going to be bothered either way, so I pick what means the least work.
Do we have a 2.5 licensed article? I don't think we can legally have that, considering SCP in itself is 3.0 licensed… unless that thing is really old. Like, editthis-wiki old or older.