You cannot simply enter a new a fake cda number and 'fool' SE int
unbranding your phone, there was an unbranding hack going around tha
would secretly send a undbranded cda version code to sonyericsson an
sonyericsson would unbrand your phone by installing the new firmware o
the unbranded cda version.

sony soon caught on, and put an end to it.

Believe it or not the cda version is unencrypted, it's written on
.txt file in A:\system\data\cda_version.txt

Inside that file the following is written

CDA162004/3 RCA01

If you change the contents of that file to something like

CDA162003/5 RCA01

and then run the online updater you could theoratically unbrand you
phone.

Here comes the difficult bit however,

Changing the ROM files are impossible, because they are all read onl
and there is no way of changing it unless you have some sort o
flashing tool, I've managed to 'browse' into the phone using sman
tool, but you can't edit any files, simply view them