To ensure the stability of the entire power grid, it is necessary for grid-connected power converter to flexibly change its operational mode upon demands to adapt to complex and varying operation conditions. This paper thus proposes a flexible control scheme for grid-connected converters to seamlessly switch between grid-following and grid-forming modes. In this scheme, the seamless transition is ensured by an integrator and a virtual admittance control(VAC) scheme, which can effectively suppress the fluctuations during the mode transition. Simulations have been performed, which verify the effectiveness of the proposed flexible mode control, in terms of fast dynamics and seamless transition.