Will an underwrap on a 7 ft med action freshwater rod change the action or make it stiffer?

The action will remain the same, but the rod will become a little softer. Extra weight always makes a rod softer.

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Tom's right. You don't need the underwrap on that type of rod

Having said that, I should add that unless you really go wild, short, light underwraps aren't going to make a huge difference, but some difference they will make.

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Fred,
As was pointed out above underwrapping will not change the action of the rod but it will lower the performance of the rod. You will be adding weight with the underwraps which will have the same effect as using larger guides or too many guides or too much epoxy. The added weight will lower the efficiency of the rod a little affecting sensitivity and feel but probably more importantly it will lower the rods resonant frequency and tip velocity which will reduce the casting distance or the amount of effort it takes to cast a given distance.
If the rod is going to be used to cast I would not underwrap the guides. If it is a rod that will not be used to cast then the additional weight is not going to have that much effect.

Thanks guys, I don't think I'll underwrap.