Upgrading my membership

I just started playing chess, and I really want a chess.com membership. I did a bit of reading on upgrading your membership, and it seems like if you buy a platinum per se, you can upgrade it to a diamond for a few months with your credit from the platinum membership. I'm not sure if I will stick with chess, so I don't want to buy a year of diamond, and then not use all of it, wasting lots of money. Is it possible to buy a year of platinum, and then turn it into a few months of diamond, or just buy a year of diamond, and then get the money back for the months I don't use?

Side note: To clarify, is each lesson just one little part of each topic, or one topic of lessons? For example, Opening Principles says it has 5 lessons, but each of those are so short I was wondering if they were even considered a lesson. Thanks!

Depends on how you purchase. Through the website is  a little more flexible but if you buy a year of Platinum and then want to upgrade to Diamond, the unused Platinum will be credited to a new purchase of Diamond; if you get the year, then it will be a year from that point.

The site offers a 30-day money back guarantee, but I don't think you can use 6-months then get a refund for the remainder, but that ultimately would be a question for support.

If I upgraded from platinum to diamond right after I got platinum, would it count the first month as used?

The credit is pro-rated. So, it will depend on how much is used of the Platinum.

First of all, wow, you answered fast. Second, what do you mean by "pro-rated"? 

pro-rated means if you consume platinum for 6 months and then you want to switch to diamond,then the half amount will be a cut from platinum and will be applied to diamond membership

 Gold is the one with the most benefit because it literally has all the things that Diamond has but cheaper.