As for the issue of older reviews with a different rating system: How about simply converting the old ratings to match with the new rating system, like how Youtube did when changing its 5 star rating system to a Y/N system? For example, a rating of 8.2 could be rounded to 4 stars out of 5, a 9.6 could be rounded to 5/5, a 3.0 could be rounded to 1.5/5, etc.

You would still get the .1 differences in average rating, but forcing people to choose between 10 ratings (5 stars with 0.5 differences), would be mean more accurate reviewing.