That is not a change in visa, but Adjustment of Status. And NO, fiancé won't qualify to apply for Adjustment of Status within the US. IF you are US citizen, and she returns to her own country upon termination of L1b, then you could apply for fiancé visa (K-1). Visas are only issued outside the US.
IF you marry, and you qualify, you can apply for Adjustment of Status to get her a green card. She must maintain her status on L1b visa throughout the processing, about 6-9 months. If she leaves her job, or if her visa is expiring, she must return home. Then you could file for spouse visa (CR-1).
So how much longer does she have on her L1b visa? Would she be able to continue in her current job & visa long enough to process an Adjustment of Status if you married? No marriage, no AoS. As only a fiancé, she goes home & then you can file for fiancé visa.