Hi Zommes,
I will suggest create one console application which connects to your CRM organization.
1.Get All the Accounts/Contacts records where Checkbox value is checked.
2. Get this Checkbox Value.
3. Update your OptionSet field (dropdown) based on the above value.
Hope this helps.
If you find this post helpful then please "Vote as Helpful" and "Mark As Answer". Thanks and Regards, Mohammad Yusuf Ansari
http://microxrm.blogspot.in