I've been under this impression for a long time too. But, I have a contradicting DP for my Marriott Premier that I upgraded to the Marriott Premier Plus. The card's AF anniversary is 6/1. Last year's AF posted on 6/1, I paid $85, upgraded it in October to the Premier Plus. Got a refund for 7 months worth of AF (makes no sense to me since it had been 5 months), and the new AF anniversary is now 3/1. Thinking this might have also reset my card anniversary for the free night, I SM'd Chase and was told my card anniversary is still in May which was the original opening month of the card.
So basically I ended up paying $36 for 6/1/18-2/28/19 and used my old Cat 1-5 FN certificate last year, received 50K pts for the upgrade, and should receive my new up to 35K FN certificate sometime in June or July, for the new $95 AF.
Maybe this is what happens for co-branded Chase cards vs Chase UR cards? I can only go based on my own experience. I think
@Judique had a similar experience with her upgrade IIRC.
I'm curious about if I upgrade my one of my CFs (I have 2, one is a downgraded CSP) to a CSR after my CF original card anniversary in 8/2019, if I won't be charged the $450 until the next card anniversary in 2020... Would you expect no $450 AF until 8/2020 in this scenario? Then, I could use up my $300 CSR travel credit in Jan (CSR anniversary month) and downgrade it in Feb. Hmmm...