I disagree with the solar company's opinion.
I find nothing in the law or instructions to form 5695 that specify that a home may only receive one credit. If you install new equipment (new panels, new capacity), I believe that qualifies for a new credit.
However be aware that there are no credits for maintenance or repair. It's also not clear to me that removing a failed system at the end of its life and installing a replacement would count as "new", but that's a problem for another day.