I recently upgraded from Lightroom 2 to Lightroom Classic (Release 9.4). Photos that were imported with Lr2 exhibit a different set of Develop controls under the Basic gorup then LrC. If I remove and reload the photo using LrC, the controls are consistent with the new release. While this works for piecemeal editing when I'm looking at a catalog of 50,000+ photos, that's not really feasible. Does anyone know why the version under which the photo is added to the catalog matters and if there's a way to modify the catalog additions from Lr2 to conform with LrC (or a good reason not to do it) in bulk job or at least in groups without reloading them? Thanks.
Now that you have switched to LrC, new imports will automatically be processed using the current process version. Previous imports, images that you have been working on will remain at the process version that was used previously. If you choose, you can switch them to the current version in batches by highlighting them in batches or Highlight all photos in the library module and go to the calibration section and choose the current version (after selecting auto-sync) and apply the current version to all your previous images. You may not need to do that. Apply the current version to a few images and see if it benefits your choices and decide from that what you need to do. You may not want to start over and redo all your editing. But that is a decision that only you can make.
There are significant changes from Lr 2 and to LrC 9.4 to process version, profiles, and much more which could significantly change how the images will look. You may wish to retain the look of your original edits. Adobe made the decision to retain your original work. New imports will be imported with the latest process version and profiles.
I would select random images and change the process version and see what changes take place, before deciding to change all. You are likely to get surprises.
It's possible the lot but the volume will take some time, do not expect it to complete instantly.