I was involved with doing it once, but it wasnt that large. (Customer changed their mind). We had to draw up new plans, get them stamped for on site reconstruction. If yours isnt living space you might get away without the stamp (depending on locale) but it's a good idea either way. You may need to add a beam for one, or sister the floor joists, etc.
Hard to do much more educated guessing without a drawing, local codes, loads, pitch, etc. When the original trusses were designed and what load criteria was used would make a difference as well.