identifying a surface mount component can be tricky, but once you get used to doing it, it can get easier. SMT devices come in all sorts of sizes and packages. Resistors and capacitors will come in varying sizes: 0201, 0402, 0603, 0805, 1206, 1210, etc. The number correspond to the length and the width. Small signal transistors and diodes can be packaged in SOT-23, so markings on the package may help determine what it is. Diodes can also come in two pin packages as well. Larger transistors can come in SOT223, DPAK, D2PAK and others.
Surface mount resistors can come in a number of different colors (they are not color coded, though). Many times it is black. Package sizes of 0603 and larger may have the resistance value on them. Ceramic capacitors will be a brown color and there really is no easy way of determining what they are.
Replacing them is easy - I'm sure there are tutorials around (not sure if the one on this forum cover smt)
replacing an smt with a leaded device is possible, but you will be introducing extra parasitics which may affect the circuit - depends on the circuit, of course.