Alan wrote:

Is it true that when you modify the end of a recurring appointment
series, it resets any of the appointments in the series which had been
individually changed to a different date/time?

It's true and the reason why is because you are changing the original entry.
Recurring appointments are not multiple calendar folder entries. Each
recurrence is generated from the original and not actually separate items on
your calendar. Instead, the exceptions are entries in the original that
modify the calculation. By resetting the end date, you cause Outlook to
recalculate all the recurrences, setting everything back to what it would be
if this were a new entry.

If so, is there a
workaround to prevent it?

Don't add exceptions to a recurring item, make new items for the exceptions.

Also, how can I change the default end from "no end date" to, say, end
after 10 occurrences or on a certain date?

I don't think you can modify the default.
--
Brian Tillman