Q: If a musjid was not a musjid and become a musjid, i as a woman was informed that a woman can go into it during her menses.
A: If a building was formerly used for another purpose but was later designated as a masjid, its previous use does not change its current status. According to the majority of scholars, a menstruating woman may not remain inside the masjid. Scholars permit her to pass through or enter briefly when there is a genuine need.
This ruling applies only to the area formally designated as the masjid. A menstruating woman may enter areas that are not part of the masjid, such as a classroom, office, hallway, lobby, courtyard or multipurpose room. She may also enter a musalla or prayer room that has not been formally designated as a masjid.
The determining factor is not whether the building was originally a masjid, but whether the specific area is currently designated as a masjid. Your local Musjid administration should be consulted regarding the building’s official designation areas.

Allah Certainly Knows Best.