Companies can change titles at will, as long as there isn't an employment contract in place. It would be rare to find a contracted title outside of a union setting. There isn't a lot of public humiliation involved either; even it feels embarrassing.
You told everyone that you got a promotion. Friends, family, and clients know. Heck, the public relations people were even involved. Do you know how many of those people care? Your spouse, your parents, and you. And those people only care because they love you and want you to be happy. Everyone else? They don't spend more than three seconds a year thinking about your title.
Now, all that said, this is unacceptable behavior on the part of your HR head. The mistake was made on their end, and they should fix it there. That fix should not involve you reverting to your old title. And now, here's a little secret: HR is not the boss.
Sure, lots of people think HR has the final word on something like this. But, they don't. Management often concedes to HR. It's a nice excuse. “HR said no.” Say that, and we all shuffle back to our desks, cursing the bad HR person. While in this case, your HR person is behaving badly, they can be overridden. Maybe not by your direct supervisor, but there is a chain of command, which means somebody can overrule them. If you want to dispute a job title change, you have a number of steps you can take.
Accept the Title Mixup
Even though we're trying to figure out a way to help you through this, winning is not the only possible outcome. So, first, if you can get to a place where you are okay with the job title change no matter what happens, you'll sleep easier.
Talk to Your Boss
If your boss wanted to promote you to a job with the word "chief" in the title, he should be able to. Your boss is not a first-level supervisor with three years of experience. You undoubtedly report to someone pretty senior.
Go to your boss and say, “HR just told me that my title wasn't formally approved. Since I was introduced to the new title at the trade show last week, and PR has officially publicized my promotion. It will be pretty embarrassing for the company if they have to backpedal. What do we need to do to get this approved through proper channels as soon as possible?”
Note, this dialogue does a few things. First, it doesn't mention your embarrassment at all. You're making it all about the company. Won't the company look silly if the title is changed? This is important because senior leadership people are very concerned about how the company is portrayed in the public eye. They care less about your feelings (although good managers do care about your feelings as well).
Second, if you state the problem this way, you're assuming that the chief title is the right one and it's just a paperwork issue. You're not asking for a promotion or title change. You're asking how to fix the problem.
Find Out the Real Issue
HR people are typically super busy and don't care about silly things. So, if your boss gets pushback, find out why the title of "chief strategist" isn't working for the head of HR. It may be that everyone at your pay level is supposed to have a title starting with associate vice president (AVP) and it's a higher grade for chief. If that's the problem, change to AVP and let it drop. However, if the real problem is that they wanted to be consulted and wasn't, then you go on to the next step.
If your boss cannot or will not escalate this, you can escalate it yourself. Go to your boss's boss and do the same thing. Since everyone signed off on this the first time, it should fix the problem.
You can continue going up the hierarchy until you reach the HR person's boss. This may be the CEO, it may be the CFO, or it may be someone else. But that person has the power and authority to override an HR decision.
What’s important is to recognize that HR is never the boss and that you are still doing the same job even with a different title. Because titles vary so much between companies, there isn't even a lot of difference on the resume between a senior director title and a chief title. What's important is your accomplishments.