If you’re a Sheraton owner looking to upgrade, I’m not sure I understand why you’re asking your question on a Wyndham forum. Are you wanting to purchase an additional timeshare week? That would be adding a ts, not upgrading. However, if you are wanting to know more about purchasing a Wyndham ts on the resale market, you are in the right place. With Wyndham, you can either buy weeks at a specific location or points which give you access to any of Wyndham's resorts.
Timeshare owners who want to get out of their contracts because they can’t travel any more, can’t or don’t want to pay their maintenance fees will sell their timeshares here on TUG Marketplace, ebay, and other sites. Some even give them away. You have to be careful because there are scams but there are also great deals to be had.
So, you need to decide where you want to travel, when you want to travel and how much money you want to spend. Browse around here on TUG. There are forums for all the major ts companies and ask questions. I’m m in the process of purchasing my first resale contract. I spent a great deal of time here on TUG just reading the sticky’s and forums to learn about the process and what to look for.
Good luck!