Why was the traditional character 滾 simplified to 滚? It's only one stroke less so it seems doubtful that it was done to make the character simpler to write. Does it change the meaning of the character in some way or is there any other logical reason why this character was simplified in this fashion?
"Simplified Chinese" is not the clearest name, because it refers to the entire standard character set of the PRC. Although the majority of characters different in the PRC compared with other regions are indeed "Simpler" (by measure of stroke counts), this is not true of all characters. Some examples where the PRC standard have more strokes, whilst still being called "Simplified Chinese", are「强」and「榨」(other regions tend to use「強」and「搾」, which both have one less stroke).
Formally, a "Simplification" process is an active attempt to cut strokes off characters, by removal, abbreviation, or replacement of components. This is not the only way that characters change, and in fact the vast majority of characters did not change in the direction of "Simplification"; many did not change at all, and among those that did change, the majority became more diversified over the years, with components added to create new characters to express new words or extended meanings.
Choosing variants which just happen to have less strokes, as is done in「滾」vs「滚」, is not "Simplification" -「滚」was merely a variant chosen as part of a standardisation process.
The component「袞/衮」(Emperor's ceremonial robes) is comprised of semantic「衣」and phonetic「公」. However, this does not mean that in「袞」,「厶」was changed to「口」. On the contrary,「公」originally looked like「㕣」.
「公」originally depicted a pottery jar, and this word is now written with variants「瓮」and「甕」.「八」depicted the handles of the jar, and「口」depicted the jar's opening.
Squares and circles were not the only representation of this opening, and later on,「口」was subject to shape variations, with some becoming triangles or stylised into「厶」.
Not a real answer (i.e., a total guess), but too long for a comment:
Zhongwen.com (a useful resource for issues etymological) says that the traditional character 袞 ("imperial robes") comes from the characters for official (公) and clothes (衣), with the bottom part of 公 altered to 口. As many simplified characters are based on common alternate/handwritten forms, it is possible that the variant with an un-altered 公 survived because it appears to match the meaning of 衮 better than the form with the 口. The simplification committee may have followed that precedent when deciding on the simplified form.
(The simplified form of 滚, of course, would naturally follow.)
『滾』 is the formal form of gun3. 『滚』 is just used in normal people (like 『閱』『門 + 兊』(yue4)). This difference could be seen anywhere before. These days there are still many differences between Traditional Chinese and Simplified Chinese like 『強』『强』(qiang2). (I am not sure 口 is formal or 厶 is formal.)
Simplified Chinese just chooses the forms written by normal people（『俗體字』） first. So then, 『滾』 is simplified to 『滚』.