Yes - you can go to 10.2 and keep SQL as your storage.  The migration of SQL to a Cassandra / Elastic cluster can be done at any time after the update, and the data migration can be done automatically at run-time.  So the actual upgrade to 10.2 and the data migration from SQL to Cassandra / Elastic are two separate steps that one can take.  Note that support for SQL will be terminated on 10.3.x, slated for Q4 2022 (Third-Party Software Support Life Cycle - DataMiner Dojo).   So a data migration will have to be planned before moving to that version.

IMPORTANT:  Skyline has announced that as an alternative for a private data storage (Cassandra / Elastic) that can be deployed on-prem or in a private cloud by the user, it will be working also on a Data Storage as a Service solution provided by the DataMiner Cloud Platform.  In that case, through the DataMiner Cloud Connection (which already provides out of the box capabilities such as Dashboard Sharing and MS Teams ChatOps), one can use the DataMiner Cloud Platform as the storage for all data (rather than having to deploy, setup and maintain your own storage cluster).  So for users interested in that, it might be better to hold off on the data migration, and keep 10.2 on SQL and re-evaluate the options when Data Storage as a Service becomes available as part of the DataMiner Cloud Services offering.