If you've ridden say 2000 miles and are going to do 1000+ more during the tour, I would replace the chain, even if it isn't worn out to a serious degree. If serious wear occurs during the tour (which seems likely), it might damage the cogs, which would mean you would have to replace those early. The bike mechanic might say the chain is still good or he might advise to have it replaced - I would ask for a new one regardless.