Y Plate to J Plate

I'm retiring in Okinawa and will lost sofa status. How do I change my Y plate to a J plate? I checked the JSVRO site but it is a little confusing. Can the process be done alone or do I have to have a special power of attorney? Thanks for your help.

This is not "Y" plate car city web but process is same.
http://www2.kek.jp/rso/Living/AutomobileRelated.html
Click purple menu bottonsall. (except レンタカー/Rent a Car)

But in this case No need attorney(弁護士/bengo-si) power.
If cannot be by yourself call/ask to (行政書士/gyousei-syosi) office enough.
"Gyousei-syosi" is one of lower class(limited) lawyer but translate to EN a bit hard.
"Certified Administrative Procedures Legal Specialist" is too long as calling.
https://www.gyosei.or.jp/information/service/
(Not only related law of Car/Vehicle)

Anyway ask at City/Town hall first is the fastest.