Hi Pmista,
A new
Official Guide has been released each year (going back several years) - and with each new version, approximately 10% - 15% of the questions are removed/replaced with new questions. The OG13 and the GMAT2015 had the exact same content, meaning that there are been 5 'sets' of replaced questions between the OG13 and GMAT2020. Unfortunately, I don't think that anyone has tracked changes between books separated by that longer period of time (each year, we create a document of changes from the prior book to the current one - for example: GMAT2018 to GMAT2019 and GMAT2019 to GMAT2020, but nothing longer-range than that). There's no harm in redoing questions that you've already answered - but if you see a question in the GMAT2020 that you already worked on when you saw it in the OG13, then you can feel free to skip past it. Between those two books, you are going to see a LOT of practice questions, but you have to be careful about confusing "quantity" of study with "quality" of study.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich