The better approach would be to ask if the company has a BYOD policy; where you can supply the device and they would provision it with the data plan and any company management/policies required.
The Problems to be overcome otherwise:
A general perception, if not actual, breach of trust with the company. This can affect you both personally (your own perception of your integrity) and professionally. I'm not you, nor am I your boss, so I can't say how this comes out; but you should at least give some consideration to both. It's way too easy to back yourself into a moral and professional corner.
Trust, once lost, is almost impossible to repair. You could be taking a few company pens home, and no one would care; until you lie about it. Then you are a lier, and who knows what you might be doing.
It's not always fair, but that's life.
The data plan is on the SIM, but the SIM cards on the iPad 1 and iPad 3 are different sizes. While it is possible to cut down a larger SIM to fit, if you make even a small mistake the SIM will become useless; at which point you either do without a data plan or fess up to the company.
Getting a new SIM would be possible, if you owned the data plan. Since you don't, you can't.
If the iPad is also managed by the company, it probably has policies installed. In that case changing the SIM would only change half the things you need to change to keep the iPad working properly with the companies' servers, email, etc.
I realize I'm being preachy, and it's not really any of my business. But it really comes down to what you are willing to do, and risk, for the sake of having a newer iPad for work.
