Depends on what you mean by the new version.
If you just care about the graphics, you can enable the “old” graphics option in settings. While you can play with the old graphics, all of the new reforged changes will apply. Here’s a list of stuff removed from the new client. Reforged is a forced downgrade for classic users
This list isn’t complete and we are still finding out things that have been removed. If you want to play the classic client, please refer to my post above. So far doing it yourself is the only way, as Blizzard doesn’t offer a classic tft client anymore.