Just wanted to say thanks and share my experience in case it helps others.
After discovering that I couldn't find any IP phone (i.e. Skype or similar) that would dial an international free number (++800 number) which AC use for their web reservations, I had to use my dad's landline as I don't have one myself.
Expecting to have to have a long wait, I was pleasantly surprised to get through almost immediately and spoke to a great agent who was pretty helpful. The change from LIB to SJO on my return flight was immediately accepted with no issue, the outbound flight he gave me was reasonable - it was the best available on Air Canada's site in terms of timing though the 2nd leg is on Avianca.
Overall it wasn't so bad. Who knows if this will be the last change, but thought I would post my experience if even just to clarify that the LIB/SJO change wasn't an issue.