Want to know if there is a collective word to describe these kind of words that change their meanings in an opposite way (rather than irrelevantly) when a single letter is added/removed/changed so that I can look them up. Otherwise can you provide some examples here? e.g. appeal <> appal
A whole class of words have this characteristic on the strength of their having adapted the Greek alpha-privative prefix into English, such that a- means "not." This arrangement leads to single-letter-based meaning reversals in word pairs like biogenesis/abiogenesis, chromatic/achromatic, gnostic/agnotic, historical/ahistorical, moral/amoral, political/apolitical, sexual/asexual, synchronous/asynchronous, theistic/atheistic, tonal/atonal, and typical/atypical.
We have various labels to describe the insertion/deletion/alteration of sounds within a word: Epenthesis (related: infix), Elision (related: disfix), Syncope, and others. But these concepts generally apply due to morphological or prosaic reasons. However, given your example of appal and appeal, it seems to me that you are asking about words that do not change by mere affixation or contraction. Otherwise, @SvenYargs gave a perfect example of such a class of words.
I do not believe there is a named and identifiable class of antonyms which differ by a single letter. My reasoning is that such a class of words is too narrowly defined to be meaningful to linguistic analysis.
That said, your example of appal and appeal is no less intriguing. I first discounted their relevance because their Old French etymologies are unrelated. However, they are ultimately derived from Latin, and their Latin roots also differ by a single letter, though in different way:
- appeal - from Latin /ad + pellere/ 'to beat or drive'
- appal - from Latin /ad + pallere/ 'to turn pale'
This last bit may not add to the discussion, but it is curious that these Latin antonyms differ by a single phonogram. Might there be others?