How the heck do you upgrade to Master Tier without cancelling your subscription? And if I cancel, do I lose all my current content?

So you already have the Hero tier?  Canceling that subscription will temporarily limit your access to 6 characters (which you pick, I believe).  The others are still there, just not accessible.  When you then purchase the Master tier subscription, your access will be restored.  

I'm just a fellow subscriber, though, so you should probably visit the site help link and see if there are clearer/better explanations.

I took the risk a few hours ago just to see what would happen, and everything went down pretty smoothly. I was super worried that the whole thing would poop the bed. I guess we can consider this answered! Thank y’all for replying. 

I'm glad everything worked, and I'm about to do the same.

That said, does dndbeyond really want to cause fear when people think about giving them more money? Huh? Do they?

Not a good idea, imho...

Better to provide a clear, simple, obviously safe path to do so.

I think there needs to be an upgrade button for upgrading from Hero to Master Tier

From your keyboard to someone who makes real decisions eyes! I bought the legendary bundle and now I have to upgrade.

I contacted customer service and was told to cancel my subscription and resub to Master Tier. I did that and still have access to all of my content. I will update when refund comes through.

The reason why upgrade isn't offered at a touch of a button is probably because pro-rated refunds is a complicated process that is best dealt by a person and done manually than by a bot that can screw up the timings due to calendar issues or leap years or such things. It's also so the system cannot be used to pay then cancel immediately over an over in order to confuse a system - every payment and refund D&D Beyond must process requires drawing on resources, routing through external software which contacts bank servers, and involves a lot and they have to pay money each and every transaction. Having it as a manual process reduces the possibility of browser and user error or the system being used maliciously (a pricey way, but possible).

It's a very minor inconvenience on your part that you will probably only face once or twice ever. It's not worth the heavy investment in time and money to implement a difficult system that can be more easily exploited and more prone to errors all to save a very few individuals a very minor inconvenience of a few minutes in a lifetime. I'd rather they stay as is and focus on features that will actually make a difference and for the better.