Craft Commerce uses a conversion factor/rate to figure out how to price the others.
Once you add more than one currency to Commerce, it becomes a bit more obvious as to what's going on:
Once you set the Primary/Base currency, every other price will be derived from the base.
So, if you change the primary base currency, you're really only effectively changing the name and the conversion factor/ratio will also be off.
As noted in the docs, Commerce also doesn't keep that conversion rate up-to-date for you.
How to Change the Base Currency
To change the base currency, you would effectively want to change all your prices. However, that's not too difficult—you'd need to export all of your SKUs with the current pricing, change the base currency, and the conversion factors, then reimport the prices at the new base currency's rate.
You'd also want to update any shipping or tax rates as well if necessary.
One thing to consider is the store's base currency is the value you put into the CMS. However, you can show whatever currency you want to the user on the front end and show it as a "default".
If it were me, I'd choose the primary currency that the store's location uses or whatever you use to manage your books then expand out as necessary.