Hello, my friend.
First, it is certain that the GPON ONT cannot be directly converted to the EPON ONT. The difference between EPON and GPON is that EPON and GPON use different technologies. The optical modem cannot be directly replaced due to different technologies.
As shown in the following figure, this is a GPON ONT.

If you need to convert GPON to EPON, you can only replace the ONT.
Thanks!