Re: Difference is casting rod wts ?
If you are going from a 5 to an 8 or 9 wt, I would not worry too much about it.
It is when you are going way up to a 10 or 12 wt where the problems exist for even good casters, if they suddenly are doing a lot of casting as in surf fishing or blind casting with a 10 wt or heavier.
As Turbine said, fishing too light a line into the wind or with really large flies/poppers can be very exhausing. Once you get over 60 years old, it can also result in pulled muscles - I know that from experience. It is akin to trying to throw a wiffle ball as fast as a baseball , as I learned as a kid.
Even though I fish year round and the smallest rod I use is a 7wt, I do isometrics for my backcast muscle and my hauling muscle daily for a month or so before tarpon season every year. I do a lot of blind casting with a 12 wt for tarpon, and don't like to have those muscles so sore after the first day that it hurts to drink coffee the next morning. But I have never pulled a muscle when using a big rod.
Casting the rods, I think, is not so different, so long as the rod actions are the same.