False. Conjunctions and prepositions can not be interchanged
Take the word "for". It is a preposition, as well as, a conjuction and the two greatly differe. You cannot change the two types and maintain the meaning of the sentence.
Ex. George gave the cake to Rebecca.
George gave the cake for Rebecca.
The first sentence tells who George gave the cake to, but the second sentence does not and changes the meaning.http://wiki.answers.com/Q/Conjunctions_and_prepositions_can_be_interchanged_true_or_false