How to edit permanently G code? LightBurn

1. That new focussing plate is probably heavier than what it is replacing, so in order to keep the workload the same for the stepper motors (so as to retain positional accuracy), it would be necessary to reduce acceleration expectations.
For example, if $120=2200 and $121=1800 (x and y acceleration), and if your original mounting bracket is about 4/5 the weight of the new one, then the acceleration would need to be reduced by roughly 1/5.

To do this, you can go into the LightBurn console window 1 and enter (one line at a time):
$120=1800
$121=1500
$262=400 (this is the threshold for shock and movement increase to 400 to reduce sensitivity)

2. Size change due to extension - go into the LightBurn console and enter
$131=390

Going from your picture, it looks like this new Z-mount requires the Y-home switch to be extended out a little to allow parking space for the increased overall size of the laser module + mount, so at the other end you would have to reduce Y Max travel by 10mm.

3. Not quite sure what you mean here, but if you want a helper-grid marked out on your waste board?, then you will have to edit and re-cut it at a reduced size (if it was a 400x400 previously).