I agree. H.S. diplomas 14 years out of high school are not a big issue. Take the promotion. Go for the glory. Don't say a word. Your time on your job demonstrates your stick-to-it-ness.
However, since you seem to have such a strong sense of right and wrong, do yourself the favor of getting that GED - even if you opt not to take this promotion. (You'll be ready next time.) Don't update your employee file when you do. You said you had one: after getting one, you'll have one. What's to update unless you're asked directly WHEN you got it?
Remember, you don't have to score high, just average. How were your grades when in school? What is your i.q.? Do you do any reading now? I mean, besides looking at the pictures? Unless your reading comprehension/vocabulary skills are really below par, I recommend picking up a kid's math book, reviewing your fractions and decimals, walking in and taking the next scheduled one. Believe me, unless you freak at the very thought of 'testing', it is not hard and you can take it as many times as you need, though there may be some required period of time between tests. If you want to see where you stand on your skills prior to taking it, call your local community college and ask if you can take an entrance exam. The two are much the same and the college can interpret your skills relative to those a h.s. graduate should have. Think clearly, don't lose your cool and MOST IMPORTANT, read the questions before reading the paragraphs so you know when you've found the required answer...saves time and the reading of confusing, miscellaneous info you don't need.
I dropped out of school after 8th grade. I passed the GED the first time without study. My first college entrance scores showed me at a level of post-BA graduate. (No, they didn't give me the degree but, they should have.) So, it can be done. Haven't you heard statistics concerning the idiots our h.s. readily hand diplomas to?
You might search the Web. There must be practice tests out there somewhere.
Good luck, I hope you get a nice salary increase. Hey, do you ever pay for free advice?