“How do I change the Type of an existing membership from say, subscription to trial (or vice versa)? How do I change the Limit field from Limited to Unlimited (or vice versa)?”

You can not change the type of an existing membership.

“Existing membership types seem to be "Available for" the following: "Male, Female, field_value_sex_4".  What is this last one?!”

You just removed a value and a key for one of the Sex values. You should remove it properly here: Admin Panel>Profile Fields>Sex>Values for Sex field

“Where do I define whether I send a message to people when membership status changes?  In "Global Configuration > Additional Settings" I can see "Send welcome letter on registration" and "Send notification, when profile was reviewed" but I cannot see anything like "Send notification, when Membership Type changes".  Am I missing something?  Is it one of these others? “

Such notifications are sent automatically. You can modify text of the message here: Admin Panel>Languages>Email Templates

make it possible to CHANGE the characteristics of a Membership Type

This can not be done. These changes may lead to different errors.

make it an option to NOT send a message when the Membership Type changes; for example, I want to change all of my existing members to a different membership type, but I do NOT want to send them a mail AT ALL.

We'll not include this feature into the software default version. But you can contact our soppurt staff regarding this modification.

Hello - im thinking about buying the unity software
and read your thread with interest.

If Im reading this right then I think this is a no-brainer standard feature - I mean all membership
sites should have ability to offer the members the opportunity to upgrade to
different membership types at any time - otherwise
the business is missing a major revenue growth stream.

In fact I would not buy software that did not provide such an important
and fundamental tool for growing the membership through type upgrades.

Perhaps Im just wrong and have misunderstood.

can anyone advise?