Situation: The cleric of Pelor was held against his will, and forced to become a vampire. Pelor is known for especially hating the undead, but also "urges his followers to remember that excessive attention to things of evil can blind one to the truly important things: compassion and goodness" as Wikipedia says. The cleric is also repentant and remains devout.
In the monster manual, vampires are "always evil", however it is noted that "always [alignment]" simply means it is exceedingly rare to find a creature of [not that alignment]. Let's say the cleric is able to stay within 1 alignment step of Pelor by some miracle (pun unintended).
Questions: Does anything happen to his spell casting? Will he need to find a new deity that's willing to accept undead followers?
(Inspired by the question https://rpg.stackexchange.com/questions/82431/implications-of-cleric-becoming-undead, which was closed as unclear.)