What happens if I run Pixhawk 1 f/w and not Pixhawk 1 1M, on a 1meg Pixhawk?

I finally managed to upgrade the FW on my Pixhawk 1 using QCG, (latest version of MP makes it impossible for me to upgrade it on a Mac) and my hexacopter flew fine on this firmware (4.0.3). I just spotted a 1M version of this firmware in the firmware list in QGC, and indeed my Pix is one with the 1meg issue, so I am guessing I should put the 1M firmware on it. What is the risk if I don’t put the 1M firmware on a 1M Pixhawk?

It should refuse to accept the 2Meg version. Please use the 1Meg version. It has less features, but it will work.

I have managed to get fmuv3 (~same as Pixhawk1) on a board with 1Mb and “internal error xxxxx” was shown in the messages screen occasionally. Of course I never tried to fly with it, just on the bench.

It certainly never complained when I installed the non-1M version. I know the Pixhawk definitely has the 1M erata issue as I recall carrying out the test in the original NuttX shell (we are talking a few years back now).