Well you could add the top fronts, and call the old ones top rear.(you cant add as TF/TM, or TM/TR. there has to be a “space” between them. This is a tough one for me, because I would personally try and go with TF and TR and either cover the existing holes, or leave them installed, and buy a flagship AVR to use TF/TM/TR.
iirc the original question was if adding two more, use the TF or FH location. Imo, you’d probably not benefit much from adding either. I think the imaging, and “bubble” would be shifted forward, and the TF/FH might just get lost during playback. I guess as I think about this while clicking away. It seem like I’d just leave the ones already installed and try it out. Where is the couch? Is it up against the back wall etc...? This is important to know. If you have some room behind it, you could possibly back it up and get the IC speakers a couple feet in front of the couch. That would be closer to spec anyway. And no, I wouldn’t use IC speakers for FH locations. That would make them much too far off axis.