Normally, when people say "EPROM" they mean the UV-Erasable things. Is that what you mean here?

For practical in-system or in-application programming, you need Flash.

The trouble with Flash is that you can't be executing code from a flash device whilst programming that same device (uness it's a special dual-bank type).

The trouble with the 8051 architecture is that it has no instruction to write to code memory - so you'll need some means to re-map your Flash device while you're programming.

So adding in-system or in-application programming to an existing system that was never designed for it could well be tricky...!

You also need to think very carefully about how you're going to guarantee that the new code is good before you blow the old code away - especially if you're doing it over GSM!