My initial answer is no.. Because Bitlocker is not a part of the lower end flavors of 7 and must be poresent. You should decrypt the data first before downgrading the OS
However, with that said,,, there is the policy you can set within Group Policy for USB flash drives to "Allow Access To Bitlocker-Protected Removable Data Drives From Earlier Versions Of Windows" that can assure that you have access to the USB Flash Drive on other operating systems and computers.
I do not know if this will work on External USB HDD's. I have not played around with bitlocker a whole lot,, need to set some time aside for testing it out.
The Group Policy for this is as follows
In the start search type Group an then click on Edit Group Policy
Computer Configuration / Administrative Templates / Windows Components / Bitlocker Drive Encryption / Removable Data Drives
This policy also shows under Fixed Data Drives. But only mentions FAT file system, if you formatted it as NTFS, I am not sure it will work.
and there you will find the policy.
This policy settings configures whether or not removable data drives formated with the FAT file system can be unlocked and viewed on computer running Windows Server 2008, Windows Vista, Windows XP with SP3, or Windows XP with SP2 operating systems.
But I doubt the data will be recoverable if you downgrade to Home Premium prior to unencrypting it.