I have two GFI outlets in my kitchen. I am replacing them w/another color. The new color are only available in 15A, while the ones I am replacing are 20A. The only thing I plug into them are toasters, coffee pots, and mixers... not sure if the lower amp has anything to do with the ground fault protection. Any ideas?

On the same topic, can I replace a 15A light switch with a 20A switch?

Q1: You would need to change the breaker too to a 15 A.
You could just replace the breaker with a 20 A GFCI breaker and use a 15 or 20 A standard outlet.

Q2: Switches rated at a higher amperage is always OK.

What you can use is based on wire gauge: 12 AWG and 14 AWG can have 15 Amp breakers.

12 AWG usually has a 20 amp breaker

Switches have to be rated equal or higher than the load.