Don't you have the required o-ring to complete Core One to One+ upgrade?
You can print it from TPU95
O-ring ID: 25mm, tube diameter 3.5mm, for spoolholder-static-r1 in CORE One/+ printable parts, CORE One+ specific parts https://www.printables.com/model/1167816-core-one-printable-parts/files
Steps https://help.prusa3d.com/article/prusa-core-one-to-core-one-upgrade-community-version_979820 

Pokud nemáš o-ring pro Core One+ držák cívky (při upgradu z One an One+), můžeš si jej vytisknout z Prusa TPU 95A. funguje :-)