I am not 100% sure this is the story you are looking for because it does not include contact with the origincal TCB Twilight, but it does contain contact with A TCB Twilight.

The crossover story is Words of Advice from the many worlds of Twilight

The Conversion Bureau series the above story intersects with is Negotiations

The specific story that links the 2 is Reaction

I hope this helps.

I get so annoyed often at the 'crossovers' with TCB, over half of them seem to like ths "ALL newfoals are shells! Celestia and twilight are extra super duper evil!!!!!" interpretation