It probably is too late to just change the chain without having major skipping on the smaller most used cogs. I would measure the chain elongation as DaveSSS recommends. If it's 12-1/6" or so and there are no skipping problems, just keep using it. If it measures 12-1/8 or more, change it and the cassette to keep from wearing the chainrings excessively.

I have a Shimano 9-speed chain on one bike with 8000 miles and it measures exactly 12-1/16" over 24 pins. I'm going to change it reasonably soon but it's still shifting and running fine.