It is necessary to upgrade TMs for use in Studio 2019.
When this is done, a new TM is created. The default setting for the language resources of the new TM is Studio's default language resources (see image). If you wish to use the language resources of your existing TM, this must be set manually in a tab which is not actively shown to the user during the process.
I cannot see a use case where you would want to upgrade a memory but not keep its segmentation rules etc. These were set for good reasons and the upgrade is being performed as a necessary step to use Studio 2019, not because there has been a change to the translation process.
I would therefore suggest that the default setting be changed to retain the language resources of the existing TM as standard.