Physically Handyman, no difference.
Grat ratios are very different.
Diff crownwheel is easy enough to swap but the pinion is part of the output shaft and that's no so easy. It's a full egarbox strip job.
Have a look at
this website and have a read of EvilAlly's post in the Peugeot Section.
Between us, you'll find everything you need to know about getting inside a BE3..
Have fun!
EDIT: I can also supply a very big PDF of the BE overhaul chapters from a very old BoL when they did go into detail. It's actually for the BE1/5 fitted to early BXs but apart from reverse being lift up by first, they are the same.