Past USACO Problems

In terms of preparation in solving USACO gold problems, (currently when there are 4 divisions, plat, gold, silver, and bronze) how would silver/gold problems (when there were only 3 divisions) be comparable to current gold division problems? In specific, would the previous silver division or gold division have problems of the same difficulty as current gold? Would past years silver problems be more beneficial to solve, or would previous years' gold problems be more ideal to prepare for the current USACO gold?

Dynamic Programming and Contest Prep

Also as a side note, could someone provide links/resources to learn dynamic programming (dp) because that is my weak point. I understand the nature of dynamic programming problems, however, I often struggle to come up with a solution that builds off the optimal substructure. Additionally, what else would one recommend to perform well in USACO gold contests, apart from learning algorithms and practicing previous USACO problems?

Thanks for all the help!