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I looked all over the forum to locate a way that case converstion would change case'S to case's.
Or any word that has a 'L (capital letter at the end of a word) (wasn't he's that's)

Any help will help

create yourself an action of the type case conversion (see FAQs /t/967/1 on how to create an action)
and then follow the FAQs on how to create an action to modify cases.
/t/967/1

Whe creating such a case-conversion-action you can specify which characters delimit words - if you leave out the ' then the ' would be treated as any other character and would not cause a case-change behind it.

Concerning your signature: shouldn't "we have lost and missed placed along the way" be "we have lost and misplaced along the way"?

Here is an example of five possibilities to change the casing anyway using Mp3tag functions.
Pick the one you need.

For demonstration only I use the preview feature of the Converter 'Tag - Filename'.
Converter: Tag-Filename [Alt]-[1]
Formatstring:

$caps('case''''S')' -- '$caps2('case''''S')' -- '$caps3('case''''S')' -- '$lower('case''''S')' -- '$regexp('case''''S','\L+\>','\L$0')

Result:
Case's -- Case'S -- Case's -- case's -- case's

Same thing with another test string ....
Converter: Tag-Filename [Alt]-[1]
Formatstring:

$caps('It''''S a case''''S case')' -- '$caps2('It''''S a case''''S case')' -- '$caps3('It''''S a case''''S case')' -- '$lower('It''''S a case''''S case')' -- '$regexp('It''''S a case''''S case','\L+\>','\L$0')

Result:
It's A Case's Case -- It'S A Case'S Case -- It's a case's case -- it's a case's case -- It's a case's case

Modified example using regexp ....
Converter: Tag-Filename [Alt]-[1]
Formatstring:

$regexp('It''''S A case''''S case','\L+\>','\L$0')' -- '$regexp('It''''S A case''''S case','''\L+?\>','\L$0')

Result:
It's a case's case -- It's A case's case

DD.20110815.2015.CEST