Will the new (vNext) version of TFS IP support the database and sessions of the previous version? Imagine we have active sync sessions and we would like to upgrade to the vNext version without loosing them.
Yes. Setup for the tools carries a delta from the last released version to the latest one and it uses extended properties on the DB to figure out what you have. It will play the steps necessary to update the DB while preserving your data.
As always, backup before an update - but yes, this should work fine.