I just read up a bit about ASM since we don't use it, at least our oracle guys just use the raw volumes they get and only tell us the lv names they want to use. We didn't had the request yet to change major or minor numbers.
I tried to find any info about this, but couldn't find anything about it. It seems they always just create the devices with the major numbers you need, like in this article:
Creating ASM devices on AIX « AskDba.org Weblog
.. and also there is something about it in this one, but just when creating:
http://www-03.ibm.com/support/techdocs/atsmastr.nsf/5cb5ed706d254a8186256c71006d2e0a/f939ff19b6430df7862570e5005027dc/$FILE/10gRAC_ASM.pdf
I think you can't change them with chdev as it is no attribute you get listed with lsattr for example. I have not worked with ASM yet to give you more info, sorry.
Not sure if it's sufficient just to have the VGs with same major numbers like you can get when importvg them with importvg -V <mj-number>... as when you are importing VGs by same major number inside a cluster on different cluster nodes.
Maybe one of the others have more experience with that, sorry.