You got three main options:
1. Contact the hardmodder and ask them if they can undo it.
2. Update, which may brick your wii (I have no idea on this issue, so don't do it until someone else posts more info on this).
3. Softmod over the hardmod (yes you can!) and I'm pretty sure you can install cIOSs which will allow you to play games requiring a newer firmware.
Obviously, option 1 is the best, but DO NOT DO OPTION 2 yet as it may have a 100% chance of bridling your wii, for all I know.
Sorry if I didn't give enough info!