There won’t be a forced upgrade to matrix/CE19. We urge people with capable hardware to upgrade and in the end of the year we’ll terminate support for the legacy builds as announced here: CoreELEC 9 (Leia) End of Life Announcement
There aren’t many (or any) main addons that are still incompatible with matrix. So I would urge you to upgrade. The upgrade is pretty simple, either burn a new image to the emmc if you have an emmc USB/uSD adapter or after resetting your installation just upgrade with the CE19 tar file.