[Change Strength] allows you to change the truth strength of an assertion, making it either :default or :monotonic. It does not allow you to change the truth of the assertion, i.e. you can't make a :true assertion :false.
When you click [Change Strength], it prompts for the new truth strength. When you click the "Change Strength" button it queues the operations to unassert the old assertion and reassert it with the new truth strength.
[Change Strength] is seldom used since the system chooses the truth value of an assertion automatically: GAFs which begin with the predicates#$isa and #$genls are monotonically true, while all other assertions (including rules) are default true.