The following statement
SET optimizer_switch = REPLACE( @@optimizer_switch, '=off', '=on' )
as well as many other similar constructions, fails with ER_WRONG_VALUE_FOR_VAR: 1231: Variable 'optimizer_switch' can't be set to the value of 'table_elimination=on'.
In 5.2 and 5.3 it works all right.
It does not seem to be about the value, since this way it works:
SET @a = REPLACE( @@optimizer_switch, '=off', '=on' );
SET optimizer_switch = @a;
But not this way
SET @b = @@optimizer_switch;
SET optimizer_switch = REPLACE( @b, '=off', '=on' );
It also does not seem to be about the result of REPLACE being too long, since this one works:
SET optimizer_switch = REPLACE( @@optimizer_switch, '=on', '=off' )
REPLACE statements are a convenient way to set optimizer_switch to a needed value in tests, and keep it version-independent; otherwise, it's not an important problem in itself, but it might signify an underlying issue, either with the REPLACE function, or with the optimizer_switch, so I find it worrisome.