I'll add a notion, that Aurora tonns are not weight measure, but volume measure - it's a mass of liquid hydrogen, displaced by craft's hull, and there is no such thing in Aurora as above-water part of ship.
So, 20% - is 20% of hull's volume, that's not adding some heavy stuff inside the same hull.
I don't know a difference of volumes between F-16XL and F-16Cs, but I doubt it's much greater than 20%, the same as DC-10's / KDC-10.
And it's the purport of refit: you have to make some changes, but not to redo your hull.