There is a way round it but it is quite a process, obviously you can't do it by the normal code change way and won't be able to do it by the program mode way as you need to enter your original code.
So there's two ways to do it try this way first, enter the 4 digit engineer code (factory set to 1234, press prog button, all the zone lights should be lit if so then enter 07 the zone lights will disappear, then press prog button again, the system will chime to confirm all the settings have reverted back to factory settings and reloaded the NVM (non volatile memory) if this way has worked then the user code will be 4321 if this way works then you would just do the normal steps for a new code change but obviously without the number 6 in your new code.
If the above process doesn't work then the other only way is a factory reset which means accessibility to the main panel i.e.. (where all of your alarm cables/power supply is connected)
Right so if there is a electrical protection supply for the alarm i.e. an electrical spur (which you should really have installed if it was done by a combatant person) switch this off to kill the supply to the panel if not open up the main alarm system box,(please take care as there'll be a live electrical supply connected, to the bottom left there's a transformer and just above to the right of it is where the quick blow fuse for the supply which is in a little black/grey pullout holder carefully pull this out which will kill the mains supply, then disconnect one of the battery leads anyone will do black or red.
Then connect the battery lead back and within 5sec press and release the tamper spring on the pub board 3times each time you press and release it you should hear a beep, after the 3rd press the alarm should start up beeping as if it would if you were if you were entertaining the house/premises i.e.. entry/exit mode, then quickly go to the keypad and enter the factory user code 1234 the alarm should stop, then the tamper circuit will flash as you have the lid off the main panel this is normal. Go and Replace the lid back being carefull as if you lift it on and off you will set the alarm off as the tamper circuit/spring is still active if you do accidentally do this just enter the user code to stop the alarm, once the main lid is on go to the keypad and press reset button two times and this should clear the tamper light and only have the day/unset/ power light on.
Then just do the the normal user code change process using the 1234 code.
Sorry for the waffle on but I wanted to explain in detail as things can easily be missed.
Hope this helps please follow back and let me know how you get/got on.