I am also going to throw out some info that might be relevant to you...
In my lifetime I have seen many employers lose senior employees due to their own foolishness about pay grades and other issues. What you really need to ask yourself here is how productive is this employee? Can you replace this employee with ONE lower paid employee, or is this the kind of employee that it would take two or three new ones to fill her shoes? Or if that isn't the case, is this an employee that currently can troubleshoot machinery issues that would otherwise cause you have down times and have to call in outside technicians?
If you can actually afford to lose her, then lowering her to the highest hourly rate for her position and then paying her overtime for overtime worked is less risky. It still clearly has risks, as have already been outlined, but its less risky. If you really cannot afford to lose her, then adjusting your pay grades or creating a new pay grade for her position, to take into consideration that she is the most senior employee, would be smarter from a business perspective.
Once upon a time, many moons ago when I was young, I worked for a very nationally recognizable company. My employee number was 77 so they obviously were not very big when I started there, but by the time I left, six years later, they had thousands of employees world wide...and are very big today. HR was always grousing about my pay, which was partial base salary and partial incentive. The most common lament from HR was that they could bring in somebody new to do my job at 1/2 of my pay. Several times my incentives were "adjusted" to bring me down to the level they thought I needed to be at, and every time, I exceeded goals sufficiently to bring my pay up rather than down. On top of that, I was doing half of my boss's job as well, without anyone realizing that. I wasn't the only employee like that, I was one of many
....and I wasn't the first one they lost...nor the last.
One of the reasons why I decided to leave was because I was more than a bit fed up with that. As it turned out, nobody they hired could do my job at my level, and it ended up with them having to hire three people to handle what I had handled...and led to my former supervisor getting demoted and then fired.
The point I am making here is not that I was anything special...because this employer had lots of employees like me. The point I am making is that employers with rigid rules about pay grades often lose sight of employees that they really need and cannot afford to lose, and lose them because of foolish rules. They also often end up paying less productive employees more money than they deserve, due to seniority and those same rigid rules.
So...you need to evaluate your rules in light of this employee, and probably others as well.