Core files modification

[ExSpirit]

I need some changes in code files (some SQL calls, display in admin order list,...) but the problem is now when I updated VM all those changes are overwritten.  What can I do, so that next time I won't have to manually fix all those modifications?

[aftertaf]

apparently you can write your own PHP helper files and override, in the same way as template overriding. just saying : i dont know how to do this but i got a reply to a similar hack i had to do...