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 GPON ONTs need to be replaced, not only GPON ONTs need to be replaced with EPON ONTs, but also OLTs need to use EPON ONTs.
You are advised to contact the device vendor to provide a solution.
Thanks!