Aahh - poor Lulu, and poor you - what a decision to have to make!
My cat, Cassie, (gone now, but never forgotten ..) went in to have a couple of teeth taken out and came home with only a couple left!!!
It was a different situation to yours - she was a persian, and her teeth were growing at just about every angle imaginable, so were particularly difficult to keep clean, so she suffered terribly with gingivitis etc..! Anyway, she was absolutely fine after a while - once her gums had healed, she was back on the feline t/d (big biscuits to make sure they are chewed before swallowing!) I tried her on so many different foods that I thought would be easier for her, but she liked what she liked, and she managed fine with next-to-no-gnashers! As I said, obviously, it was a completely different situation to yours, but she was also only a couple of years old at the time! They're amazingly resilient, and sometimes I wonder if we give them their due credit?!!
Fingers crossed the diet thing works - Hope all goes well.