To get everything pushed towards poser12, it can become a tedious task, but that's doable.
For example, each character (with its clothes/equipment/whatever) can be exported of poser11 using File > export > partial scene  and then reimported in poser12 using the "import poser document" function.
And don't forget that as long as you aren't rendering using both poser's at the same time (except under some specific conditions), both can run in parallel, so that you don't have to close one, reopen the other, reload stuff, and so one.

This is a way, another is to store in a dedicated runtime structure each item separately. But this is a long term solution, and it is for another use.

In the case of Windows, and if your system drive has enough space, your PC can handle both at the same time, even if it may take some time because of memory swapping, depending on the RAM your computed has. It' not that difficult, as poser is rather good at this: I have an external scene with a swimming pool, a 2k hdri and 13 victoria4's each with its 3 4k diffuse maps + 3 bump maps, and poser12 needs less than 5Gb of RAM, as long as I don't launch the render (it then can jump up to 18Gb...)

𝒫𝒽𝓎𝓁

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(っ◔◡◔)っ

👿 Win11 on i9-13900K@5GHz, 64GB, RoG Strix B760F Gamng, Asus Tuf Gaming RTX 4070 OC Edition, 1 TB SSD, 6+4+8TB HD
👿 Mac Mini M2, Sonoma 14.6.1, 16GB, 500GB SSD
👿 Nas 10TB
👿 Poser 13 and soon 14 ❤️