Hi, and welcome back!
Lower ISO means that the film needs to be exposed to the light for longer, so your film was overexposed.
First, let's see how bad off your film is. Photographers measure that in the number of stops you were off by, and the formula is:
log(400/200)/log(2) = 1
You were off by one stop. That's equivalent to overexposing each picture by 1 stop, which is not very much.
Your main option would be to tell the photo lab to use "pull processing" when they develop the film. This, though, will produce negatives that are harder to print, lower quality, and grainier.
Most people only recommend doing this when you are off by 2 or more stops.
However, if you are very concerned with these photos, you can ask the lab to do a very slight amount of pull processing. According to the Telus site below, you might want to ask for about 30 seconds to a minute less development time.
Here are some links:
I hope this helps,