How can I extend a deck post on an existing deck?
They make a metal brack to span the old 2X4 and you most likely can get it to work for the new roof.  If not I can most likely get some made at work.  You do have some good bracing and support on the one footing you have pictured here and if the other one is the same way it would be just as strong.

I would just pull out the old footings and replace them with the new style footing and be done with it.

Reasons:
1) you would always have questions as to how strong the modified brackets (ie the roof/posts are) and 
2) By replaceing the old footing /posts you now have done the job and will pass and inspector.