If you are going to use TestStand then I would suggest getting enrolled on the first of the NI TestStand courses. If you cannot do this, then go through the various examples supplied and read the Using TestStand ... manuals. This will give you a reasonable understanding so that you can create, edit, run TestStand sequences.
That would depend if you have changed the interface to your DLL. ie changes the inputs and outputs
If you haven't changed this, then no you shouldn't have to change your TestStand sequencefile. Unless off course the data you pass to the Dll and receive from the DLL needs to change.
If you have changed the interface then you would have to selected the steps that uses this DLL and press the re-load the prototype button. Then you will be able to make the necessary changes to your test sequence(s) to correctly interface with the new DLL.
I am assuming your system will have the correct LabVIEW RTE installed.
Regards
Ray Farmer