Dll References

Question

Hi all,
   I have one web application with only one page say "Default.aspx" with a user control say "UserControl.ascx" and cs file for that user control is just to display the message and applicaiton is hosted IIS. After the deployment i will not have that .cs file but instead it will have the dll file  say "Sample.dll"  to point to that .cs file from Default.aspx and UserControl.ascx.


   Now i will need to make a change in that usercontrl functionality, instead of modifying the existing project and replacing with new Dll, I will have to made the same set of code in new project like previous one with necessary changes and generated the new dll say "Sample_New.dll". 


 Now i have to add new dll in the previous project's bin folder..
Here how can i make only the UserControl  to point to the "Sample_New.dll" instead of "Sample.dll"  ?

Note: Only the user control has to point to the new dll, but the aspx page has to point to the old dll only.

Can anyone help me about this ?
Thanks in advance....