My bike has 53/39T chainrings and a cassette with 12,13,14,15,17,19,21,23,25 sprockets, but I like to ride up steep mountain roads. Is it possible (without great expense) to replace two sprockets to make it a little easier to spin instead of mash?

My rear derailleur is a Shimano RD-6500. I tried checking online, but I couldn't find a clear description of what the maximum sprocket size it supports is. Would it be practical to replace, say, the 23T sprocket with a 24T and the 25T with a 27T?

Or is there something else I should be considering to make the gearing a little more friendly for spinning on steep roads?