So, i might change my hard drive disk to a solid disk on my mac. Before I do that, what should I do to have my bitcoins safely saved from the hard disk to the solid disk once I install the solid disk in my mac?
You have to copy your wallet files, which contains the private keys. In case of Bitcoin Core, you need to copy a file wallet.dat. In case of electrum, you just need a seed, unless you also care about transaction history labels and saved "contact" addresses, than you should backup your default_wallet file.
Your bitcoins aren't really in your wallet. Your wallet contains the private keys that allow for access to your bitcoins. Maybe export coins to an online wallet?