Is it practical to replace two sprockets on a cassette?
No, you will need to get a whole new cassette, the largest 3 rings are connected to each other, you cannot just purchase individual cogs. You may want to get a new chain as you will probably need to make a chain length change with jumping from a 25T to 27T or 28T largest cog.

Do you know if you have the SS or the GS?

With the SS (short cage) you can go as big as 28T, Shimano sells a 12-28 cassette.
With the GS (long cage) you can go as big as 27T, look for a 12-27 cassette.

Another option for easier spinning is a new compact crankset, Shimano comes as a 34/50, but you give up high end speed.

EDIT:

Added screenshot of the relevant tech info for the RD-6500. Hope it's helpful. 

  

Although the derailleur has the capacity for 28t as an SS derailleur, Shimano never produced a 9 speed 28t cog. Therefore, as a practical matter, the 27t is the largest option available for the RD-6500 Ultegra 9 speed derailleur from Shimano. There are cassettes available from Interloc Racing Design which fill this missed niche.