WordPress MU

i customize the user control panel ... remove feeds of wordpress and change a bit of colors ... then i upgrade wpmu to newer version(whenever it comes out) ... will it overwrite all the changes ? or can there be other problems ?

yeah, it will. You have to document (for yourself) the changes you made to whatever files, and then when you upgrade in the future, make those same changes again, if needed.

and i can customize the upgrade code too ... ?
( rather customizing again )

You can maintain your own changes like andrea_r said, by documenting them.

You could upgrade other files and check what changes have been made to your modified files. Much of the upgrading has to do with keeping the code in line with the current wordpress version.

The upgrade script for the site mainly deals with database changes as far as I know.

and what if database has been splitted to multiple database ?

One way you can ease your pain is to contribute back your customisations. Those that you package well enough may make it into the code base, saving you the hassle next time.

http://codex.wordpress.org/WPMU_Commonly_Edited_Core_Files has the files many people change. These should be configuration options such as the default feeds you mentioned. If you were to contribute patches to make these configuration options stored in the database then you would not need to change the code each time.

I don't know about your multiple database question.

http://codex.wordpress.org/WPMU_Development_Methodology

WHAT DO YOU THINK ABOUT THIS?

I have arround 10 hardcoded files. So, would it be stupid to replace latest MU version files (same files of course) with those hardcoded files from previous MU version i had installed... of course, if they weren't changed in new version. That way I wouldn't have to spend hours adding those hacks and probably do some mistakes. Maybe Donncha is the most qualified to answer this question. Thanks.

EDIT: pumpkinslayer has already mentioned something similar

Keep track of your changes, then upgrade, then re-add those hacks back.

That's pretty much the procedure.

I keep a list of changes in MODIFIED.txt. In that file,
- I copy the URL to any locations that guided me in the changes.
- Put comments about exactly what was changed and why
- In the changed files I'll sometimes put
## MODIFIED: was {some code}

I've also changed the permissions on the changed files to not be (over)writable. I haven't tried it yet, but I figure that I could upload a new version of WPMU over the old one, and the failed transfers would remind me which files to check/compare.

It would be pretty sweet to have a way to automatically upgrade this stuff without mucking up the changes. Perhaps an upgrade method that can tell when a file in the new WPMU didn't change between versions.

SVN does all that.