The same two letters will change each word and phrase below into a new one. The letters are in the same order, but are not consecutive.

ILL
RISK
LIGHT
IN ALL
ELICIT
LATTER

HINT:
One of the two letters is placed at the front of each word or phrase, and the other at the end.

Answers:

The letters are F and Y, with F at the front and Y at the end:

FILLY
FRISKY
FLIGHTY
FINALLY
FELICITY
FLATTERY

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
F - Y UPON IT