Sorry, this is a repost. I did not think I had put this up here. So many things going on in the remodeling project!!

If indeed the raised portion is part of the base that supports the chimney, then lowering it becomes a big job that would basically entail tearing it down. If you are converting to gas, the rebuild wouldn't have to be as substantial, but it will cost money, including a metal flue. Unfortunately, it is likely that the raised portion is integral to the structure...if you have someone jackhammer it out the whole thing could collapse.

I am interested to know what you ended up doing. I am taking down my hearth and am interested in lowering my fireplace and converting it to gas as well. Would love to know if it's possible. Thanks.

We removed our 1 brick high hearth and old cast iron wood insert & - RAISED the gas insert to the top of the opening. We will cover the existing brick floor to ceiling with marble and a marble hearth level with the floor. We are waiting for our tile guy. Took us a long time to decide what to do. It is already such an improvement w/ the brick toe -stubber raised hearth gone & the clean face gas insert installed. The extra space under the insert (created when the raised hearth was removed) will be marble - we are using 12 x 12.