"Colin Powell went to the UN to press the case for war, and it was entirely about WMD. Why didn't he advocate forcible regime change to install a democracy?"

IF the cease fire agreement that Iraq signed after losing the war against the US coalition which ejected Iraq's miltary from Kuwait, had a requirement for Iraq to become a democracy. And Saddam had failed to take steps that would bring about a democracy, then in this kind of situation, Powell might presented evidence showing this lack of compliance with the cease fire agreement.

Imagine a situation in which Saddam had complied with cease fire agreement. Meaning say by, oh say, 1996 Saddam had worked with UN inspectors so that inspectors could come to conclusion that the Iraqi regime had complied with cease fire agreement. The result would have been that the UN sanctions would been lifted.
This would also have meant that Clinton would not had bombed Iraq in 1998. It means Iraq would not need the Oil for Food program- Iraq could freely sell oil on the market and import goods for other countries. It's also reasonable to expect that before the 911 time frame, the No fly Zones over Iraq would been lifted and that the US was longer using Saudi Arabia to base air operation. [Obviously, Bin Laden couldn't then add the bit about the infidels having a miltary force in the Saudi Arabia as part "the reason why" they conducted the 911 attack. But he could have simply added a few more reasons to the list to make up for it.]
Anyhow in this situation, we wouldn't have attacked Iraq.