We cant convert xp from a ship into free xp until that ship is elited (all blueprint upgrades complete).
For tier x, it takes a thousand games or so to get blueprints for elite status (at least that was the estimate a few months ago). So when you finally get your tier x elited, you should have generated perhaps 1,000,000 xp.
When your tier x becomes elite, can you then access that circa 1,000,000 xp for conversion into free xp? Or does the xp counter get reset to zero?