Could you please advise what are the things which get migrated from OJS2.4x to OJS 3.X?
Do article XML and pdf files move with auto migration? In addition to this, the previous user credential will transfer successfully? Now we are going to do for 10 journals merged in one OJS and it is of 150 gb in size. I am not sure what all information will migrate during auto migration process.
The upgrade from 2.4.x to 3.x will preserve the existing data, including article galleys, users/credentials, etc.
There are a few cases where data from 2.x is still present in the 3.x database, but is no longer exposed as it was in 2.x. For example, comments and user biographies, which were heavily subject to spam abuse, are not publicly visible after the upgrade.
Some less-common functionality is no longer (or not currently) available in 3.x as compared to 2.4.x. For example, LDAP authentication is one plugin which has not yet been re-written for 3.x. Overall, however, you will find significant new functionality within 3.x, in a far more modern architecture.
The look and feel and the interface of 3.x has changed substantially from 2.x. I strongly recommend trying an upgrade in a dev or pre-production environment, if possible, perhaps just to familarize yourself, or as part of user training for your journal managers and editors.