I'm relieved to have read through this and got some answers. Thanks all! ^
I purchased some Avianca miles late last year, and I was sure they were going to expire in November. But now I know I can just buy more to keep the current ones alive. :-) Or transfer some in from a hotel program....

Since any addition of miles to the account will prolong the validity of all of them, would that include canceling a ticket and having the miles redeposited? Not planning it, of course, but just curious. :-D

Now that I know I can extend them, I'll be looking for the next big sale! Think they'll have one before November? :-D