Now prepositional crossovers are what I think of as words that weren't always prepositions, but at some point in the history of the language became recognized as functioning recognizably and immediately as a preposition functions.

Many prepositions crossed over from other words, too. For example: near and next (to) are originally the comparative and superlative forms of nigh and are still used as adjectives. The re in the subject line above is a Latin noun in the ablative case (res 'thing, matter, case') meaning something like 'in regard to, in the matter of, concerning'. Sometimes when a word changes its syntactic category its just a common and normal situation. For example, nobody would complain of turning verbs into nouns with suffixes like -ing / -ed), but if somebody turns architect from a noun to a verb, some people get bent out of shape. Lexicographers tend to be on the conservative side. Not all neologisms or slang terms make it into the dictionary. It seems like concerning has been being used as a preposition since the 15th century.