Hi Craig,
Since Midsize Business and Enterprise plans are in the different service families, Midsize plans and E3 plans cannot be purchased in the same organization tenant. So, the InfoPath forms created in E3 plans couldn’t be seen in the Midsize plan.
For more information about changing plans or subscriptions, please check the link below:
office.microsoft.com/.../can-i-change-my-office-365-plan-or-subscription-HA104031833.aspx
Thanks,
Nancy Xie