I had come across a situation where i needed to convert Umbraco 6x using mysql to Umbraco 6x  using mssql. 

I followed the following steps and it seems to have worked.

1. Use https://www.microsoft.com/en-au/download/details.aspx?id=42657 to move the contents from MySQL to MS SQL

2. Create a new empty blank Umbraco database in MS SQL server of the same version as the mysql site. (Use 'Tasks --> Generte Script' option in SQL server on a pre existing  database with 'schema only' option)

3. Export data from SQL server created in step 1 using the ‘Tasks --> Generte Script' option. Make sure you set the option to ‘Data Only’ and allow identity insert in the generate script option.

4. Go the database creates in step 2  and drop all constrains on the database using a script as motioned here (http://blog.sqlauthority.com/2013/04/29/sql-server-disable-all-the-foreign-key-constraint-in-database-enable-all-the-foreign-key-constraint-in-database/)  

5. Now import all the data using the script generated in 3.

6. Set the constrains back on the database using the script mentioned in step 4

After this i used nuget to upgrade umbraco to v7x. All seems to be working but more testing needs to be done, so use this with caution.