Microsoft have a range of products with a variety of different rules & rights around what you can/can’t do with them such as downgrading to a previous version.
Even for partners, it can be difficult to know each & every license right for the different products and that’s where this site can come in very handy:
For any OEM or FPP product this site should tell you what you need to know.
Making the selections as above gets you a pdf of the full license terms:
This can be a great way to double check if you’re allowed to do something!