There is a way to migrate Work Items from one TFS to another TFS. This requires TFS to support Office 2007.
Please follow the below steps to migrate WorkItems(fox ex:Bugs) from TFS1 to TFS2 server.
1. Export the WokItems to MS Excel from source TFS from Team explorer
2. Open the WorkItems in destination TFS in MS Excel
3. Go to Add-Ins and choose the columns to export
4. Copy the WorkItems from source excel to destination excel
5. Click on Publish(From Add-In tab) to Insert the WorkItems to destination TFS
6. On successful Publish, ID appears back on excel
You can also upload the attachments for each bug through excel.