When it comes to upgrading from an i7 to an i9, there are a few things that you need to take into account. The first is whether or not your current motherboard can support the i9 processor. If not, then you'll need to upgrade your motherboard as well. Second, you'll need to make sure that your cooling system is up to the task of cooling an i9 processor. And lastly, you'll need to factor in the cost of the actual processor itself.
Assuming that your motherboard can support an i9 processor and that you have a suitable cooling solution in place, let's take a look at the cost of upgrading. An i7-9700K costs around $350, while an i9-9900K will set you back closer to $500. So, all things considered, upgrading from an i7 to an i9 is going to cost you around $150.
Now, whether or not that's worth it is up to you. If you're a hardcore gamer or video editor who needs every ounce of performance they can get, then upgrading to an i9 makes sense. However, if you're just a casual user who doesn't really push their PC hard, then sticking with your trusty old i7 might be the better option.