For example a vendor might use IBM Tivoli Omegamon to perform system monitoring and you are using BMC MAINVIEW. In this case you would be able to drop your BMC license. Since the vendor is now performing system monitoring you would no longer need to retain that license.
In other cases, a mainframe hosting vendor might have products that are similar but not identical to products that you are using in-house. For example if you are using CA-Top Secret for security and a vendor has IBM RACF. Going through this type of conversion could be a large project and it probably would not be worth the effort. Converting products depends on the cost savings that can be attained versus the effort involved in a conversion. The conversion effort involves the retraining of staff, the conversion of the software, the testing of the new product and the time that it would take to implement. Converting complex products are often not worth the cost saving that they may provide.