If your license supports it, then you can install version 9.
So if you have a lifetime upgrades license, it supports all versions of eM Client. You can see what you have by going to Menu > Help > License. If it says All Future Versions, and Pro with Lifetime Upgrades, then of course you can install version 9 without any upgrade purchase.
If it doesn’t, then it is not valid for all versions, and it will say which version it is valid for. If it doesn’t say 9, then you will need to purchase a license upgrade so you can install version 9 and use its features.