Idioms in all languages are typically univocal as to form. If English has ‘kick the bucket’ with its idiomatic transferred meaning of dying, then there is no latitude for speakers to change ‘bucket’ to ‘pail’ (or any other synonym for that matter). However, when idioms first enter a language, some variation may occur, whereby one or another synonymous element is substituted for the authentic version.
This has happened in recent American English with the idiomatic phrase ‘get/wrap one’s head around’, meaning ‘understand with some difficulty’, such that speakers frequently replace ‘head’ with ‘brain’ or ‘mind’. These substitutions ought to be ruled out of court for one fundamental reason: they are the result of a misconstrual of the stylistic link between the verb and the direct object. Both ‘get’ and ‘wrap’ are words from the common (non-elevated) stratum of English vocabulary, whereas neither ‘brain’ nor ‘mind’ is. Speakers who alter the idiom by changing the stylistic level of its verbal complement are thus guilty of linguistic misprision.