The rear support bar is completely different, to fit a T4 in an early bay t a new bar needs to be fabricated, the T4 engine mounts are quite soft and close to the middle of the engine (T4's have rubber between the engine and bar, T1 have mounts between the bar and body), in a late bay there a third mount at the flywheel end of the gearbox which stiffens it all up.
It should be possible to chnage back to a type T1 as long as the mounts which drop down from the chassis haven't been removed and altered although to fit A t4 IN WITH STOCK cooling the rear valence would need modifying.
Crossover's have a mounts the same as late bays so all the parts are available off the shelf to fit either.