someone pointed out years ago now(cannot remember who) that his circle actually changes from copper to bronze and back to copper before he ever had it upgraded. So yea, something's off about that..
A somewhat cheap fix/Retcon (edit: or explanation) is the fact that these books are sometimes considered (or speculated to be in the forums) his journals written after the fact in the tradition starting all the way back with Merlin. Harry could simply be remembering things wrong...
Oooorrrrrr... he may not have thrown the old circle away, I still have my old laptop sitting in a corner right now, I didn't just throw it away when I got this computer. I do need to look up some things though, like if his circle was embedded in the floor...
Edit: He did refer to it as his OLD summoning circle, and keep in mind that while it's been 10 years since he had the appartment to us, for him it's been ... 2, 3 years? My copy of Changes is a hardcopy and buried somewhere so I can't check that as easily as my kindle