I have a DB that collects info on what activities a student is interested
in. It's a long list and the client changes his mind on what the interests
are each year.

I have all activities as individual entries in a table:

Aerobics
Baseball
Fitness
Football
Softball
Yoga

The Activity Name of this table is used as a Value List with checkboxes in
the Student table. (I can control which activities appear in the list
through an Appears field and calc field.)

This year the client wants some activites listed in the Value List in
combination -- such as Aerobics/Yoga/Fitness or Baseball/Softball.

What I would like is a way to create a new Activity that links with the
individual fields, so that:

1) if the new combo name is checked, all linked Activities are checked.
(Likewise if you change your mind and need to uncheck it, the linked Actives
will uncheck.);

2) if the combo name is edited or deleted it will NOT affect any data
already entered in the individual Activities; and

3) if possible, if one of the linked Activities is already checked prior to
linking the new combo will be checked (to preserve the legacy data).

I figure this give me real flexibility in editing the Value list in the
future while not losing the data already collected.

That's what I want, but I'm not sure how to get there.

Any thoughts?

Sounds like you need to fake the value list by creating an additional
table and loading it with the record choices (selection list) that the
client wants this particular year. Then display that list of records
in a portal.
If your checkbox field is a calculation field then you can have it show
as checked if any of the 3 Aerobics/Yoga/Fitness were previously
checked. Clicking on the calculation checkbox would have to run a
script that set or removed the 3 words from the text field that
actually stores the data.