This thread explains how CUST works and how you can save it. It seems that the recovery can save it, but to be sure you should ask the OP of the thread.
As for bootloader unlocking,
I found this. It is a simple update.app that unlock your bootloader, then you are good to go I think.
The steps for you would probably be:
- unlock bootloader
- flash TWRP
- make a backup
- flash original recovery
- update to B133
- update to B609
If it goes wrong you can flash TWRP in fastboot mode and then use the backup you did. Also the all cust partition might handle the differences between your current and the international cust, but then again, you should ask the OP of the cust thread.