Letter exchanges indicate a certain letter in a piece of fodder and instruct the solver to replace it with another to help form a part or all of the answer word.
Letter switch indicators prompt the solver to exchange a letter in a word for another one which has been indicated separately. Here are some examples:
To get RUM to become RUN the setter needs to indicate that M must be swapped for N and so may write:
rum to prefer knight to man ...
... here, the letter switch indicator is TO PREFER. So RUM prefers N (one of the abbreviations for a knight in chess) to M (the abbreviation of man). And so RUM becomes RUN.
To get RUM from RUN the setter may spot that it is the last letter which needs changing and so use a switch indicator such as:
run to make last mass ...
... the letter switch indicator TO MAKE LAST means the solver makes the last letter of RUN an M (the abbreviation of mass). And so, RUN becomes RUN.
Any words that fairly lead the solver to the switching of letters may be employed. Here are some more examples:
Letter switch indicators - WITH X FOR Y/WITH A CHANGE OF HEART
Again, the clue being in a down or across position will affect the indicator being used.
Judge giving one to ten years for the third of charges (6)
judge - definition
giving one to - I (one) & letter exchange indicator
ten years - decade
for the third of charges - A & letter exchange indicator (the third letter of charges
So the instruction can be read as:
Get the answer by giving I to DECADE in exchange for A
(DEC(-a(+I)DE) = DECIDE
Bird society finds new leader in large contest (9)
bird - EMU
society - NATION
finds new leader in - letter exchange indicator
large - L (clothing size abbreviation & the letter to be exchanged for the first letter in NATION)