For me, Heather Tom IS Victoria. I loved her so much and still miss her. I'm not a big B&B fan so I don't watch that one more than maybe 1x/mo. When Amelie Heinle took over the role, I think the writers took to writing the character differently. Heather's dynamic with Eric Braden was different so the Victor/Victoria relationship was more interesting to me. Amelia perhaps seems more timid with Eric Braden and for me it comes across in the Victor/Victoria relationship.
I think with David Tom as Billy they will take the character in a different direction. I've already been thinking the Victoria/Billy coupling is on the way out and she'll be moving on to Ben/Stitch. I think they will make Billy younger now and age Victoria more appropriately. I feel like when Billy Miller took over the role, the writers wrote the character as more mature, experienced, older, etc. He's younger than Nicholas but Billy Miller couldn't play that young as he's more mature in general.
But then again, I could just be doing wishful thinking since I like David Tom so much and want Billy to be different... and not with Victoria for that matter! LoL.