Upgrading from EPON to GPON in an ISP

So basically I do not work for the ISP in question.

However, the owner of the ISP has reached out to me for help.

Problem:

1. They have approximately 3000+ customers on their existing EPON infrastructure
2. At least 30% of said customers are equipped with an XPON ONU that supports both GPON and EPON, the rest are on EPON ONUs
3. So this ISP wants to move everything and everyone to GPON
4. But they do not know how to go about it without running around thousands of people's houses just to switch and configure the ONUs

So what do you guys think is the optimal solution to shift existing EPON customers to GPON without having to run around?