Unfortunately there is no way to do this online. When you’re creating tags essentially what you are doing is allocating a block of memory whose size is based on the data type you choose. For example, an INT will allocate 16-bits of memory which can represent a number from -32,768 to 32,767. Whereas, a DINT will allocate 32-bits of memory for that tag which can represent a much bigger number -2,147,483,648 to 2,147,483,647. These memory allocations cannot be changed dynamically at runtime.
Hope that helps!