OK, reinstating. I'm very skeptical to this word actually being used seriously outside of manga and the like (my understanding is that the Iga museum is mostly for entertainment). Not sure whether [hist] would be appropriate or not.

@@ -14 +14,5 @@
-<gloss>dance in which the dancer quickly changes his clothes seven times</gloss>
+<gloss>dance in which the performer quickly changes his clothes seven times</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>
+<gloss>the seven disguises of ninja</gloss>

I agree that I can't cleanly identify the historical source for this (maybe ninja's don't take careful notes?) but the Iga-ryu Ninja Museum went so far as to print 七化 on a sign (photo included with hand-transcription of photo, including furigana):

https://iganinja.jp/2007/12/post-51.html
https://iganinja.jp/2008/02/15/sichihoude.JPG

忍者は敵地に潜入する時、正体と目的を隠すために変装した。変装は「虚無僧」「出家」「山伏」「商人」「放下し」（曲芸人）「猿楽師」「常の形」（農民や武士）の７つとされ、忍術伝書では「七方出で立ちの事」または「七化」（ななばけ）と呼ばれた。
A ninja could disguise himself in seven different ways: as a straw-hat-clad Buddhist priest, traveling priest, a novice priest, merchant, acrobat, street musician, or in everyday attire (farmer or warrior).
Image posted in museum December 2007.

Apparently the sign is still up, as is. Photo of same display, 2021:
https://blog.goo.ne.jp/2014kurumatabi/e/ad46f2fc24ca833161929096d8875912

Another photo from 2012 (I think this poster might be the origin...)
https://ameblo.jp/peace-study/entry-11379537800.html

Well, now that I hand-typed it, this text is currently echoed (copy/paste) on https://www.ninja-museum.com/ninja-database/?p=732
I'm not actually sure of the affiliation of this website, so the previous reference to a standing museum seems more concrete. 
七方出（しちほうで）
忍者は敵地に潜入する時、正体と目的を隠すために変装した。
　忍者は敵地に潜入する時、正体と目的を隠すために変装した。変装は、「虚無僧」「出家」「山伏」「商人」「放下師」（曲芸人）「猿楽師」「常の形」（農民や武士）の7つとされ、忍術伝書では「七方出で立ちの事」または「七化」と呼ばれた。

This appears to be a modern work of fiction, so maybe all ninja-afficionados are just passing the same anecdote around:
七方出
https://ncode.syosetu.com/n2851cy/7/
これを忍術伝書では、「七方出(しちほういで)立ち事」または「七化」といい、俗に「七方出(しちほうで)」と呼ばれた。
https://pdfnovels.net/n2851cy/main.pdf


忍術（忍者）の「七方出しちほうでの術（七化ななばけ）」は、虚無僧・出家・山伏・商人・放下師・猿楽・常の形（つねのなり、武士や農民）の7つを指す。
https://meitou.info/index.php/放下通し

A hand-drawn cartoon image with a ninja in 7 different outfits, titled 七化. The text is in Chinese. Posted by the Embassy of Japan in China?! (日本国驻华大使馆)
https://weibo.com/ttarticle/p/show?id=2309404005136853813468&infeed=1

Here it is on Wikipedia, marked "source needed"
特に虚無僧・出家・山伏・商人・放下師（ほうかし、大道芸人、奇術師）・猿楽・常の形（つねのなり、武士や農民）の七つは「七方出の術（七化）」と呼ばれる。
https://ja.wikipedia.org/wiki/忍者#.E5.A4.89.E8.A3.85

Here is the exact moment it snuck into wikipedia.  August/2009, so the source may have been someone visiting the ninja museum?
https://ja.wikipedia.org/w/index.php?title=忍者&diff=27715863&oldid=27711298

Seems like someone needs to pay the museum a visit and get a better source...

A chinese blog reference from 2006
https://blog.xuite.net/h0937397696/twblog/151787021-忍者相關知識

忍者的基本「七 化」（nanabake）是： 出家、虛無僧、山伏、放下師、常之形、商人、猿樂師。 「出家」（syukke），是一般光頭和尚。 「虛無僧」（komusou），頭上戴著圓筒形竹笠，蓋住整張臉孔 ；身上披著袈裟；頸上掛著托缽；邊吹蕭邊化緣乞討。 「山伏」（yamabusi），在山野中修行的僧侶。 「放下師」（houkasi），相當於現代的街頭藝人，表演魔術、 特技、耍猴等。 「常之形」（tunenokata），化身為在地農人或武士，條件是 ，必須會講一口流利的當地方言。 「商人」，就是賣藥、賣糖的行商。 「猿樂師」（sarugakusi），江湖藝人。 

Chinese blog, 2005
http://cg.17173.com/content/2005-1-29/n94_10658.html
首先，当然是得临机应变化身为各种身份。忍者的基本「七化」（是：出家、虚无僧、山伏、放下师、常之形、商人、猿乐师。「出家」，是一般光头和尚。「虚无僧」，头上戴着圆筒形竹笠，盖住整张脸孔；身上披着袈裟；颈上挂着托钵；边吹萧边化缘乞讨。「山伏」，在山野中修行的僧侣。「放下师」，相当于现代的街头艺人，表演魔术、特技、耍猴等。「常之形」，化身为在地农人或武士，条件是，必须会讲一口流利的当地方言。「商人」，就是卖药、卖糖的行商。「猿乐师」，江湖艺人。其它还有「变相术」、「变体术」、「变声术」等。

Typo [performer].  (Was just browsing around and happened to see it)

That said, I imagine the gloss should match the referenced entry for 七変化? (That's what I changed it to)

Stylistically I'd drop [his] and use 
  "dance in which the performer quickly changes clothes seven times"
for both entries, but I'm not going to mess with the other entry over style preferences.

Anyway, only here to warn about the typo, no personal stake in this.

I also changed the sense from [3] to [1], as the entry for 七変化 was re-ordered 2023-01-08.
~~ 
I got curious about the ninja reference and started researching the sources.  Best guess:
Some museum curator at the Iga-ryu Ninja Museum misheard/misread something somewhere, and put 七化 on a public-facing sign in a popular exhibit, which sat in place for the next 2 decades.  It gained little traction until someone thought to copy/paste the museum reference into wikipedia (August 2009, definitely more recent than the museum).  It then became a part of a bunch of students' wikipedia copy/paste homework assignments "give a presentation about ninjas".  And here we are.

Still not overly common, and I can't find 七化 in the 忍者 context, except as 七方出... 七化とも呼ばれ。 (The apparent Chinese Embassy of Japan hand-drawn 七化 entry being an odd exception...)

So, I'll leave my reference digging just in case this ever comes up again.

@@ -12 +12 @@
-<xref type="see" seq="2106490">七変化・3</xref>
+<xref type="see" seq="2106490">七変化・1</xref>
@@ -14 +14 @@
-<gloss>dance in which the perfotmer quickly changes his clothes seven times</gloss>
+<gloss>dance in which the dancer quickly changes his clothes seven times</gloss>

not seeing any evidence for sense 2 in kokugos

@@ -12,6 +12,3 @@
-<xref type="see" seq="2106490">七変化・しちへんげ・3</xref>
-<gloss>kabuki dance in which the dancer quickly changes his clothes seven times</gloss>
-</sense>
-<sense>
-<pos>&n;</pos>
-<gloss>the seven common disguises of ninja</gloss>
+<xref type="see" seq="2106490">七変化・3</xref>
+<field>&kabuki;</field>
+<gloss>dance in which the perfotmer quickly changes his clothes seven times</gloss>

'nanabake' gives supporting hits in english for the ninja thing as well

Daijr, Koj, GG5

None of them mention ninja (the original submission quoted 忍者の大常識.)

@@ -12,0 +12,5 @@
+<xref type="see" seq="2106490">七変化・しちへんげ・3</xref>
+<gloss>kabuki dance in which the dancer quickly changes his clothes seven times</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>