Some current requirements are bogus, but... if the use of a structure changes there is good reason to look it over. I've been in one barn who's broken beams were being supported by the feed bunks. I doubt in a conversion those would be there. After looking through a barn on a property a friend moved to I told him to stay out and to keep his boys out. Although it had stood for over a century it collapsed within the year. I've worked on several old homes that had joists that were well overspanned. Two I can think of were downright scary. They had stood for over a century and the angels were getting tired of flapping. We live considerably heavier now than we used to and with different serviveability tolerances. None of those sound like this barn but a critical eye is money well spent.
Google "swing beam barn" and you'll find lots of info. Here's a couple.
http://www.howellfarm.org/farm/barns/fish_barn/lessons/anatomy%20fact%20sheet.pdfhttp://www.circlerockinc.com/images/gallery/gallery-9-lg.jpg
