If you'd rather do Legal in Year 12, definitely try and switch this year if you can. Units 1&2 aren't a prerequisite, but you will find 3&4 easier if you do a semester of Year 11 and are already familiar with a lot of the terminology and assessment structure. That way, you're not still getting your bearings during your Unit 3 SACs!
However, if you're still considering doing Revs in Year 12, stick with Empires for now and you can make a decision at the end of the year. The biggest factor would be workload: Legal is way less than Psych whereas Revs is WAY more than Psych! There's only a handful of things you need to memorise for Legal, and then most of the assessment tasks are about applying your knowledge or evaluating concepts, whereas Revs has a tonne of dates, statistics, and quotes in addition to the application/evaluation/analysis questions.
If it helps, take a look through the Revs 3&4 and Legal 3&4 study designs or past papers (Legal will have a new SD next year, but there'll only be minor changes!). The most important question is which subject you find most interesting.
If you love history, essay writing, and source analysis, you'll probably have an awesome time - Revs is honestly one of the best subjects in my opinion! But if you're ambivalent and/or would rather concentrate on Psych and your other subjects, you'd probably find Legal more chill and perhaps more enjoyable.
Good luck! All the best for Psych and whatever else you choose! 😄 🧠