MLGU6 Gearbox on 2005 Citroen Relay 1500TD

[Relay1500TD2005]

Hi,

New member here looking to reduce rpms on the motorway of my `05 Citroen Relay 1500TD with 2.2HDI engine (believe to be DW12UTED)

Current 5 Speed Gearbox (believed to be MLGU5) is just too low geared so would I be able to replace my 5 speed with an MLGU6 gearbox to make motorway driving more pleasant?

If this is not possible - would the 5th gear from a 20MM01 MLGU5 swap into a 20UM16 MLGU5 gearbox?