Not all TRE's are ex-SFI's, far from that. I know a couple who were DEC when they joined. The minimum qualification to apply for TRE position is LTC iirc.
But maybe the Training Department makes everybody believe it's the 'prefered option' in order to fill the SFI vacancies??.... A lot of ex-SFI are not even selected to become LTC. And remember, after upgrade the SFI is not used as TRI by the company (there might be exceptions of course). So it's more a qualification 'on paper'...