Assign trial users to a plan that is not the Trail plan
After you sign up for an Office 365 trial, you come to a point where you may not want just the E3 trial plan for everyone, but rather mix and match plans for your users depending on their needs. To put some users on a lower P1 plan and leave others on the E3 plan do the following:
Sign into your Microsoft Online web portal (http://portal.microsoftonline.com)
1) Click on the Alarm Bell at the top of the screen to bring up the Purchase now options and click on the Purchase Now Link
2) After you get to the Purchase Now area, you will want to click on the More Offers, then check the item you want to add, and click on the add to cart. This will put the item you have selected to your cart.
3) In the cart, modify the number of users you want on that item, and check out as you would normally.
4) After you check out if you still have other users you want different subscriptions for, repeat the process. If you want the remaining users on the E3 plan (the plan the trial is for), just click on the add to cart on the first Order page.