Being an innovative member in your org can be a double-edged sword. Watch out for the edge you might not anticipate. Namely, where being indispensable because of that neat time-saver of an application you created turns into the reason people hesitate to promote you. You probably have a plus sign in the column labeled "Helps Drive Efficiency" or "Increases Revenue,", but unless you have someone capable of carrying on your work, you might get a question mark in the "Able to Backfill" column. If you're an IT worker, probably not as much of an issue. But if the standard expectations of your core role don't include any sort of MS Access knowledge, your employer may be in a tough spot.
Here's why. Your standard job role (that is, what your peers are expected to do) doesn't include a requirement for MS Access expertise, VBA knowledge, etc. So the same reason they are thinking of promoting you (you have capabilities beyond your current position) is also the challenge in finding your replacement. Namely, do they have to modify the job posting to indicate the person needs to know MS Access? And if so, do they pay them more or give them a title like "Business Analyst"?
Another potential challenge, albeit a better problem to have, will be that you do get that promotion, but still have to maintain your application. It might sound like a good compromise at first, but as soon as your new job takes hold of you, those little enhancement requests and bug fixes start to lose their luster.
So what can you do?
1) Remember that what you create needs to live on without you. The less "administrator" intervention required for it to continue operating when you've moved on, the better. That means solid error handling and messages, automation, and following best practices (naming conventions, VBA conventions, commenting your code, etc) so someone else can pick up your work and have a chance of being able to fix bugs without calling you.
2) Start cross-training. Find one or two people in your org that are sharp enough to start picking up what you've learned. That might mean giving them a crash course in VBA, table design, etc. While it might seem like it will make you less indispensable, it probably won't. It will secure your future, and let you take long vacations without worry.
3) Consider replacement. Is your solution available off-the-shelf for a reasonable price? Do you have a professional development org that can take up what you've done and replace it with a more robust solution? If so, think about building a business case to show the benefit of replacing your solution. Why? It shows you have business savvy beyond just increasing efficiency with an MS Access database, gives you an opportunity to dive into financial business cases, and may save you future headaches if you're stuck maintaining a database you created seven years ago and four promotions back (I've been there, it gets uglier and uglier to explain to a VP why you're doing VBA instead of your real job).
As always, happy developing!