Can I upgade the license type (from In-house or Vendor to Vendor or Ultimate)?
Yes, you can. In-house license can be upgraded to Vendor or Ultimate, and Vendor license can be upgraded to Ultimate. The license can be upgraded if the "old" license is for current version or can be upgraded to current version free of charge. If there's a paid upgrade involved, it needs to be upgraded to current version first.
The price of such upgrade is calculated as the difference between the cost of the "new" license and the sum paid for the previous license. I.e. if you have purchased in-house license for $1000 and now you need Ultimate license for $4000, you pay $3000.