We perform a first-principles study and identify two intriguing ferromagnets, hollow-Fe8N (H-Fe8N) and bridge-Fe8N (B-Fe8N) monolayers, by extensive structural searches. Both H-Fe8N and B-Fe8N nanosheets are buckled triangular lattices with a similar motif, but they are distinguishable by the positions of N atoms. The magnetic and electronic properties show that H-Fe8N is a low-spin ferromagnet; in contrast, B-Fe8N is a high-spin ferromagnet, which originates from the 3d orbital splitting of the Fe atom due to the low/high symmetric crystal field. Surprisingly, two stable Fe8N monolayers can be transferred to each other by N atom diffusion from the bridge position to the hollow position with the migration energy barrier of 1.5 eV. The energy barrier is affected by introduced Fe defects and rare earth metal dopants. These findings introduce a new tactic to regulate the 2D Fe-nitride monolayers at the atomic scale.