Welcome, I am Cory.
Right now, I am looking for a solution, as this was just upgraded to every account recently.
I do apologize for the delay. I am sorry to say it, but as of right now, there is not a way to go back to the old format. I do remember this same issue with Yahoo when they upgraded to "the all new mail". Enough people complained and they released a link you could use to go back to the old format, then eventually added a setting for it.
At this time, you should go on to the suggestion site located here https://outlook.uservoice.com/forums/284136-outlook-com and let them know you would like something added to go back to the older version.
I wish I had better news for you, but I do not.
Please know I have researched this and came up with the best information available.
If you have anymore questions, please do not hesitate to reply.
Please let me know if you have any further questions.
If not, please do not forget to rate my answer with the stars above.