In principle, if you are currently using FLET'S Hikari line and changing to docomo Hikari without changing the line type, a fiber-optic line installation is not required.

However, if there is a need to change the line type, etc., a paid line installation may be required.
NTT docomo will contact you regarding installation date if there is a need of fiber-optic line installation.

Reference
Charge for NTT docomo fiber-optic line installation can be found on NTT docomo's website.