沒 simplified to 没: What is the origin of the change?
&quot;Simplification&quot; is a process. If it's not reduced in stroke count, it's not actually &quot;Simplified&quot; - the mix-up in language is due to PRC's standard of Chinese being popularly known as &quot;Simplified Chinese&quot;, even though PRC Chinese may have the same number of or even more strokes.
「没」 came from a popular variant of 「沒」, because the right hand side of 「没」, 「殳」, occurs more frequently among characters, while the right hand side of 「沒」, 「」, occurs in...「沒」.

It is important to look at a character's earliest traceable meaning before further analysing its components.

Some of the earliest traceable meanings of 「沒」 is to be underwater / be submerged (cf. 「沉沒」). This is the reason for its 「氵・水」.

「殳」, if it's a semantic component, is usually used as an active verb marker, cf. 「發」 (to shoot arrows &gt; emit), 「擊」 (to attack), 「殺」 (to kill), etc. 「殳」 is therefore not a semantic match for the meaning submerge (and is not a phonetic match for the sound of the word either).


So, what exactly is 「沒」?

秦簡秦103睡虎地秦簡隸定　
「沒」 was originally a compound of a hand 「又」 under deep waters (represented by a whirlpool 「洄」). 「回」 was later gradually corrupted into the shape of 「勹」 or 「刀」.
西漢隸602銀雀山簡編三國・魏隸孔羨碑　今楷　
Note: as a consequence of this development, we can be certain that 「」 was not really a character before, but was made up to decompose 「沒」 into 「氵・水」 and something else.


References:

季旭昇《說文新證》
《王力古漢語字典》
小學堂
漢字源流