it's stringified json even lol

and yes, generally no impediment to changing it, most codecs will swallow whatever you can un-escape out of the content field, so long as it has the main fields in it

relays don't even look at what's inside the content field at all for any reason, only the tags, and only the single letter prefixed ones are indexed

changing the rest of the structure of an event is probably gonna break stuff though, but honestly, for what reason would you do this? and profiles don't even use tag fields necessarily, though often clients will populate them with any hashtags in the "about" field