Not specific to Lilypond but hope you don't mind the question, I have looked for this in online music notation glossaries without success.
I am transcribing a jazz ballad, and I am told the player thinks the solo in double-time and I should reflect that switching to 8ths rather than 16ths.
Is there a notation to mark you need to read the score that way from some bar on? and then back to normal tempo at some later bar? (I am transcribing the score of one single player, no need to sync with other instruments.)