Generally, the .NET framework does not allow you to use two different versions of controls for
the same assembly in a single web-application since
the assembly names are the same, you cannot save both of these in the same folder and also the namespaces are the same, which would not
allow you to use different assemblies on the same page or project. Thus, if you want to use one version at a time and then upgrade it later in the future, then you could save the dlls in different folders or locations and refer to these as per requirement.