What does "Wikify" really do?

Short version: "Wikify" flips a post from Normal type to Community wiki (POST /-/change-post-type, newType = 12). Two things change — who may edit the post, and how its authorship is shown. Nothing gets destroyed, and it's fully reversible. Checked against the running source (v1.2026.003) and confirmed with a throwaway test topic (since deleted).

Who can wikify / un-wikify. Staff or a Core member (trust level 6) can wikify any post and undo it (PostsDao.scala:2013-2036). A regular member can only remove wiki status from their own post — they can't wikify one (TyE0OWN6MR). The dialog wording: "Change to Wiki? … Then, others can edit it." (wikify-dialog.more.ts).

What actually changes:

• Editability opens up. The type becomes CommunityWiki (12). On this forum the default permission grants wiki-editing to the built-in Full Members group and up — verified in perms_on_pages3 (may_edit_wiki = true for group 13 on every category), i.e. trust level 3+. Staff can always edit; guests never can. (There's also a legacy staff-only StaffWiki type 11, but the Wikify button never uses it — it's deprecated, [NOSTAFFWIKI].)
• Authorship is hidden. The byline is replaced with just "Wiki" — the original author's name is no longer shown, "because this is a wiki post contributed to by everyone" (discussion.ts:1713). The author also stops getting Like-vote credit for that post. Un-wikify and the name returns.

Now your specific questions:

"History seems missing." It isn't gone — only the link to it is hidden. Edit history is still recorded and fully retrievable. In the test I edited a reply (revision 1 → 2), then wikified it: both revisions stayed in post_revisions3, and GET /-/load-post-revisions still returned the complete history with each editor named (original author and later editors). What disappears is the UI affordance — on a wiki post the header is the bare "Wiki" label, so the "edited 5 min ago / click to view edits" pencil that normally opens the revisions dialog isn't rendered. Un-wikifying restores both the author name and that history pencil.

"Allows others to change it?" Yes — that's the whole point. Full Members+ (and staff) can edit it, and every edit is tracked as its own revision attributed to whoever made it (I watched num_distinct_editors climb, each revision correctly attributed). Guests can't.

"Can have replies?" Yes. A wiki post is still an ordinary node in the discussion tree — I nested a reply under the wikified post with no trouble. Wikifying changes only edit permissions + author display, not the ability to reply to it.

"Can a reply be made a wiki?" Yes — any reply can be wikified, not just the original post (tested on a reply). Two edges worth knowing: the page title can't be wikified, and a progress-section post currently can't also be a wiki (a known "harmless BUG" note in the source — post type is a single field today, so wiki-ness and progress-ness can't coexist yet).

"…if so, does it show first?" No. Wikifying doesn't pin or reorder anything. The reply-sort comparator (ReactStore.ts:1689) orders by pinned → placed-last → deleted → votes → age — there is no wiki key — so a wikified reply stays exactly where its votes/age put it. It only sits at the top if it's the page's original post (always first, wiki or not) or if you separately pin it.

One-liner: Wikify = "let Full Members+ co-edit this post and present it as a shared, author-less wiki entry" — reversible, with edit history preserved (just no longer linked from the wiki header).