Can I change the service offering of the virtual machine after it has been created?

This is easily done. In your Stratosphere browser, find and select the virtual machine for which you wish to change the service offering. When you click on the "Change Offering" button, a dialog bog will appear and you be given the opportunity to select a new offering. The up/down arrows will open a drop down list with all of the available offerings.  When you select an offering, the description to the right will indicate how much memory and how many CPUs will be assigned.

Resizevm01

Note: The virtual machine must be in a stopped state before a new service offering can be selected and applied.