Google Voice is an inbound call forwarding service. It forwards to up to six regular 10-digit US telephone numbers, and to any Google Hangouts clients that are logged into your account and set to ring on inbound calls, and, to Google Chat/XMPP clients. OBi devices act as Google Chat clients. Google Voice doesn't care if it is forwarding to a OBi 100 or a OBi 200.
You haven't clearly stated that you are going to retire (stop using) your 100. I'll assume so.
Log into your OBiTALK portal/dashboard on a computer's web browser, and go here:
https://www.obitalk.com/obinet/pg/obhdevClick on your OBi 100 device. On the next page, click the "Delete device" button to delete it off of your account. Wait a few minutes while your 100 gets remotely reset by OBiTALK, then unplug it and do what you wish with it.
Back on the main OBiTALK web page, click "Add Device" on the left side of the web page, and follow the instructions to set up Google Voice on the 200 after it is added to your dashboard. You'll need to be logged into the correct Google/Gmail/Google Voice account that now holds your Google Voice number, and then the OBiTALK portal will set up the OBi to work with that account.
If you previously used another SPx to supply E911 service, you will need to set that up, too.