Hello rodfromsept-iles,
Thanks for your question. I understand you currently have a 23" Cinema Display and would like to know if you will be able to change the display resolution for a 27" Thunderbolt Display. The resource below explains how to see all of the available resolutions for each display you have connected to your Mac.
Using a Retina display
If your MacBook, MacBook Pro, or iMac has a Retina display, it automatically optimizes the resolution for the internal Retina display and any external displays that you attach. You can adjust resolutions for displays by choosing System Preferences from the Apple menu and then clicking Displays.
If you have Mirror Displays enabled (under the Arrangement tab in Displays System Preferences), you'll see an "Optimize for" pop-up menu in the Display tab on both internal and external displays. This allows you to optimize for the internal or external display, or even scale the content on both.
If Mirror Displays isn't enabled, then you're using Extended Desktop mode. In this case, you can set resolutions individually for each display. Check the Display System Preferences window that has the description for the display you want to adjust on the top of the window (there will be a unique preferences window for each display). Adjust as necessary.
You can see more resolution options for your external display by holding down the Option key while clicking the Scaled button.
If the resolution that you want does not show up in this list, then it is not available.
Hope this information helps ...
