I changed both the top and bottom plastic trims on my AGA 210. It was stored in the shed for a couple years, the cold cracked them both. The LFS near you should be able to order replacement pieces. If I remember, I think they were like $60 or something, and took about 3 weeks to get.
Having the tank full would make me nervous. I had to cut the top off with a roto zip. Not sure how you plan on getting the old one off, but if you cut it like I did, (peeled right off like a banana), you will make a real mess in the water with all the shavings. I hope it all works out for you. I'd be interested to hear how you changed it when you're done.
If I was in your shoes, I think I would drain it half way, clean the inside glass all around (for blue tape), and tape painter's plastic with blue tape on the inside to catch all the garbage, to keep the reef clean. Only tip I can come up with for you. Best of luck to you!