I am going to make a similiar change from 11-25 to 11-28. You should definitely replace the chain at the same time. The RD depends on compatibility, it has to be able to to carry the chain to the slightly larger cog.

To calculate the ratios, you simply divide the front by the rear. For instances 36/25 = 1.44. If you change only the rear cassette 36/28 = 1.29. The closer you are to 1.0 the easier it will be to pedal. If you only change the front to a34, then 34/25 = 1.36. If you change both then 34/28 = 1.21.

Good luck!!!