I'd say it depends on commitment level. How much they want to keep milking FGO and/or care about the LORES and attachment from playerbase. They can choose to:
- Adding more sections to servant profiles and my room lines to clarify their view of the new protag. They could go back and add Valentine scenes and voices, so I think they definitely can pull this off if they want to.
- Only update profiles, make distinction between their relationship with the 2 different Masters. This one faster.
- Make the Guda's attached servants behave like DLC characters when playing as the new protagonist.
- Pretend like they are all new summons and sometimes make references to their old experiences. This is the norm when it comes to servants anyways. No changes needed, just mute the myroom lines if it's related to Guda specifically.