I just did one too on a 2000 and couldn't get the main body to turn without fear of breaking it. I eventually just used a couple large flat head screwdrivers to gently pry it up and that worked without breaking or cracking the plastic. I only replaced the pump and when I was putting it back together I tried to pop it in the correct way and give it a little twist but it still wouldn't do it. It formed a tight seal on the new o-ring and the locking ring holds it down so I wasn't worried about it leaking. I'm more worried about the rusty tank getting a pin hole air leak in it......
2004 Sable LS Prem DOHC
2001 Taurus SES DOHC
Previously owned:2000 Taurus SE, 1994 Sable GS, 1988 Taurus LX