I've no idea when the change that you ask about occurred. The definition below from the OED has not been updated since 1982, indicating that back then it did not consider the schwa to be exclusively reserved for unstressed syllables.
A Dictionary of Psychology (4th edition, Oxford U Press) says:
The neutral and central mid vowel...that occurs in the words the and fern, at the beginning of about, and at the end of sofa, and the symbol in the International Phonetic Alphabet (IPA) that represents it, namely an inverted e. Statistically, it is the most frequently occurring English vowel (over ten per cent of all vowel sounds), yet it has no corresponding single letter in the standard alphabet. See also central vowel, formant, mid vowel. [From Hebrew shewa a mark indicating the absence of a vowel sound][my emphasis in bold]
The Oxford English Dictionary (OED) defines schwa as
The central vowel sound /ə/ , typically occurring in weakly stressed syllables, as in the final syllable of ‘sofa’ and the first syllable of ‘along’; = sheva n. 2. Occas., the symbol of an inverted ‘e’ used to represent this sound.
It gives a schwa in the pronunciation of such single syllable words as sir, purr, bird and the stressed syllable of birdbath.
The aptly named Concise Oxford Dictionary of Linguistics says
The mid-central vowel of e.g. the second syllable of matter: in phonetic notation [ə] ([matə]). Also spelled ‘shwa’.
Fowler’s Dictionary of Modern English Usage (4 ed.):
/ʃwɑː/ (shwah). In the phrase ‘a moment ago’ in unemphatic BrE speech, the two unstressed vowels in italics are pronounced identically. The technical name for this sound is schwa, and its symbol is /ə/ in the International Phonetic Alphabet. Not only letter a can be pronounced /ə/ : the italicized letters in the following show it represented by other written vowels: number, the, obey, commit, success, picture. The fact that letters a, e, o, and u can all be pronounced as a schwa explains many widespread spelling mistakes, such as *relevent.
I'll add that's it's too bad the sound of schwa is not also schwa.