Does this mean that we can install a blade module in a manned turret, or does this only apply to remote turrets?

Both could be converted to AI turrets/point defense. The restriction is that a remote turret can not be turned into a manned turret. Remote turrets like the Gladiators will be able to be slaved to the pilots control with a (blade) component installed, or made a AI turret/point defense if the required blade is installed.

Does this mean the ball turret replacement that is a fixed item mount is going away?

We don't know for sure, but best guess is probably that a remote turret can have its weapons upped in size to a single large flash fire mount, but with balance restrictions insuring that its fair/fits the ships cooling and power requirements.

It's also a little disappointing that you won't be able to switch turret types.

From the site the restriction seems to be blade space and the fact that a manned turret swapped to a remote/AI will be hit with a decreased maximum weapons load out.

For each turret you wish to be AI controlled i.e. it engages and tracks independently of player or NPC input, you need to have a Blade equipped for that. Ships that come with these types of turrets either have these blades already installed or additional computer items to hold them in, as blade space is restricted. This is designed to force players into choosing between adding this feature or other blade features when customizing your ship.