I would definitely try to keep Poser 10 and 11 installed alongside the new one, for Python 2 script compatibility among other things (13 uses Python 3 - Poser 12 and 13 scripts listed here: https://jurn.link/dazposer/index.php/p12/  Poser versions can happily coexist. Also, know that if you're still on Windows 7 or 8, then you won't be able to install paid Poser 13 scripts from the Renderosity Store (the OS lacks the required encryption module for Ken's scripts).

However, is disk space is very precious and you really need to delete... then (as well as the runtime folders) also back up folders that look like these (Poser 10 and 11 paths may vary), which contain your render presets...

C:\Program Files\ .. \prefs\RenderPresets (default Firefly renders presets, with a SuperFly sub-folder).

C:\Users\USERNAME\AppData\Roaming\ .. \RenderPresets (your custom render presets for Firefly and SuperFly).

C:\Users\USERNAME\AppData\Roaming\ .. SketchPresets (your custom Sketch render presets). 

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Learn the Secrets of Poser 11 and Line-art Filters.