According to what a Microsoft spokesperson told Neowin:
"For existing 8.1 devices, Win10 update is going to be only SW update, but in order to fix few issue, there will be FW updates for some devices which are still based on 8.1 SW before they can receive Win10 update. Right now we have not communicated what 8.1 devices will get updated and when but please have the latest SW on all your 8.1 in preparation of win 10."
As we have previously reported, carrier approval will be needed before older Windows Phone 8.1 products can receive an over-the-air Windows 10 Mobile software update. However, once that is completed, Microsoft looks like it can then push out software updates to the OS without carrier approval.