Hi! I hope some smart person can help me with this.

I recently cancelled the return leg of a flight, which left me with a credit for the unused leg. Apparently any new flight I book has to abide by the original fare rules (complete travel by a certain date.) If I make a new booking, using this credit, and then later have to move my flight, will I be bound but the original fare rules on date of travel?

Hope this makes sense. Thank you!