Scott,

2007/8/21, Wedel, Scott :
> I have been charged with upgrading a -highly- modified Bugzilla
> installation which is currently on version 2.18, to version 2.22.2.


that's a daunting task, even for people with a lot of experience with
Perl. (As a side note, let me ask why don't you upgrade to 3.0?)

I'd start off with understanding the current customizations. I'd make
a diff between a pristine 2.18 and your installation, and understand
what the customizations do. I'd try to package them by functionality
(the x functionality is implemented as changes in files a, b and c in
places a1, a2, b1, b2, b3, c1), so that I don't miss anything.
Optional (but recommended) step: find out whether all these
customizations are really useful -- maybe some of them can be dropped
because they aren't being used in reality or have been added as a
standard feature. Then, I'd see what of the diff applies cleanly to
the new version, and try and merge the rest in. Any modifications to
the database schema need to be taken into account as well.

Good luck
Marc