Complex sentence: The limits of integration are often not easily interchangeable (without normality or with complex formulae to integrate), one makes a "change of variables" to rewrite the integral in a more "comfortable" region, which can be described in simpler formulae.
Split into simpler sentences: The limits of integration are often not easily interchangeable (without normality or with complex formulae to integrate). One makes a change of variables to rewrite the integral in a more "comfortable" region, which can be described in simpler formulae.