Think back to previous discussions about the declaration of independence and the writing of the constitution. compare the meaning the founding fathers gave to "freedom" with the meaning pro slavery advocates of the old south gave the word. what changed?

The difference is that the founding fathers claimed that freedom is a right for all men and the old south didn't take slaves to be people. For the southerners, every slave was only 3/5ths of a man, so they weren't people and therefore they couldn't have their freedom and the constitution did not apply to them. That is why slavery lasted as long as it did, because people fought abolishing slavery.