Depends on your tools, handiness and determination IMO...the procedure is shown in RAVE, no need to remove the whole box, i have a friend who did it with success... for me it's too mechanical
The diagnostic I had done said solenoid valve 2 was knackered. But looking at the above diagram numbers 2 and 3 are both called the same. Is it one of these I'm looking to replace?
You need bit nr 2(THT000020) as the diagnostic showed MV2 , nr 3 in the attachment, you can find the whole replacement procedure in RAVE - Automatic gearbox - repairs