Questions about updating rooted phone

I purchased a galaxy s5 scl23, Japan version, through an eBay seller. The item was marked as unlocked however it was rooted to achieve that by the seller, in stock mode the phone is locked to the Japan AU carrier. I was unaware of this when I purchased.

My phone is thus locked on android 4.4 and I'm unable to update it through conventional means. I've researched how to update the phone through Odin however have some queries so I don't brick the phone. My understanding is that if I update through Odin to the scl23 supported lollipop firmware then I'll locked to the AU carrier network as whatever mod was done to the phone to unlock it would be deleted. I'm wondering if I can update to another version of lollipop for s5 that will also be able to remove the carrier lock? Or if perhaps I should perform a factory reset and look to update to a fresh lollipop install that will work with australian networks.

Any help would be greatly appreciated as I'm a little out of my depth with this stuff and only started researching it after I realized my phone had been rooted by the seller.

You should have sent the phone back for a refund. In fact if you are still within the 45 day timeline you should send it back and find one you can activate and use on your carrier..

I should of yes. However I've been travelling and purchased it while I was abroad, really to take photos with, so never had a sim in it until now.

According to this Australian thread on whirlpool.net.au, flashing the stock Android 5.0 update for the carrier au KDDI, mentioned above, re-introduces the service provider lock on the phone...

SCL23 - AU variant of the Galaxy S5 - Please Help - Samsung - Android phones

This tutorial on xda developers, "may", help you unlock it...

[TUTORIAL] Unlock SIM for au/KDDI Galaxy S5 SCL23