You don't have to give up your hi-lvl crews. Each of your 4 ships can have 5 crew members, in addition to their captains, and you can have an additional 5 crew members that are not assigned to any ship. You can then re-arrange or switch up any of those 25 crews to your ships as you see fit.
If you wish to upgrade your crews, you'll need to watch for when a higher tier of crew is available and then higher one. For example if your best crew member with the Seafaring trait is a Pearl Diver, you may want to upgrade to a Stargazer, if you see one for hire. You don't have to lose your Pearl Diver, unless you're at your limit for crew members. And yes, you'll have to train the Stargazer up to its highest level.
You can only have a maximum of 25 crew members, so if you have that many then you'll need to dismiss someone before hiring a new member.
As for hiring crew, if you don't like the choices you're given then you can reroll the hires up to 15 times per day. Keep trying until you get a crew member you like.
The crew available for hire is random, but there are a few things that can influence who is available. Those influences include which islands you have unlocked on your voyages, and how far you've upgraded the port lodgings.
If you don't like the crew for hire that is being offered to you, try re-rolling for a better one. You're provided with 15 re-rolls a day.