Have you got the Workshop Manual?
Wiring will be critical.
You will need to maintain all the input feeds to the ECU from the NEW transmission computer that the engine computer currently needs to have in order to work properly with the 4EAT transmission.
You will also need to use the tiptronic 5-speed computer from the Legacy to operate the new transmission. In order to get this working properly, I can see you needing to research the input-output diagram from the 5-speed in relation to differences from the 4EAT. In other words if there are additional signal sensors [or different ones!] required to inform the 5-speed computer, then you will either have to fit these to the EG33, or bluff the new TCU into thinking that it is reading acceptable signal values for these extra sensor inputs.
It needs a little bit of TCU electronics knowledge to get this working, but I see no reason why it could not be done.
Joe
__________________
Black Betty [Bam a Lam!] '93 UK spec, still languishing
Betty
Jersey Girl Silver '92 UK [Channel Isles] 40K
Jersey Girl @ Mersea
Candy Purple Honda Blackbird
Plum Dangerous
White X2 RVR Mitsubishi 1800GDI.
Vantastic
40,000 miles Jersey Girl