SmallWall and m0n0wall (and for that matter, t1n1wall) are functionally interchangeable in most ways, with just a few warnings. For the most part, SmallWall can simply be installed over m0n0wall and vice versa. New functionality that is not in m0n0wall will not be there if you revert back, but it will still function. But code signing can cause some problems.
m0n0wall was signed with a 1024 bit RSA key that is not available to other projects. This means signed SmallWall images (which now use a 2048 bit RSA key) can not be uploaded to m0n0wall. (It will show as a "file corrupt" error) And signed images of m0n0wall can not be uploaded to SmallWall for the same reason. But unsigned versions work fine. And once an unsigned version is uploaded, the signed versions of that family will work correctly.

• Download an official release of SmallWall from the Downloads page.
• Go to the System -> Firmware section of your firewall, and "Enable Firmware Upload."
• Select the file you have downloaded and "Upgrade Firmware."

• Download an unsigned image of SmallWall from the Downloads page. Most beta versions are unsigned. There will also be unsigned release versions clearly marked.
• Go to the System -> Firmware section of your firewall, and "Enable Firmware Upload."
• Select the file you have downloaded and "Upgrade Firmware."
• Select "Yes" to bypass the "Unsigned Image" warning.

- Lee Sharp