If you decided to upgrade a legendary weapon then you should stick with it for a little bit longer to make that decision worthwhile (you can lose a lot of money by selling them later). they poison enemies or increase effectiveness of your critical hits). But despite that you can decide to invest in them if they are good for your playstyle (e.g. Warning - Upgrading legendary and very rare weapons costs much more. You can do that by selecting the right option in trader's screen. Of course, you can regularly change your gear but some weapons are useful enough to be worth upgrading. This is because you can find new items with much better stats. This means that, over time, a given item won't be as useful anymore when Bayek gets a few levels (e.g.

Each item has a certain level that determines its quality.

All your Drachma should be spent on only one thing - upgrading your gear.