Hi Roberto Andrade Olivie,
Thank you for contacting me.
Setting the value of the current cell should work regardless of whether the old value is null or not. Maybe there is something specific in your project, which causes the described undesired behaviour. Please open a new support ticket and send me a small sample project, which demonstrates your scenario. This will help me to investigative your case further and provide you an accurate assistance.
the Telerik team
Q1’11 SP1 of RadControls for WinForms is available for download
; also available is the Q2'11 Roadmap
for Telerik Windows Forms controls.