I think you're misunderstanding the definition of "Max CP", which really means "Max CP for that Pokémon at your current trainer level." You can power up your Pokémon until it hits Max CP for your current level, and leveling up doesn't affect your current CP in any way.
Evolving retains exactly the same proportion of CP, so a high CP Pokémon is guaranteed to be the same relative strength after evolving.
Every Pokémon's max CP cap only applies for that current trainer level, and the max CP increases every time a trainer levels up. For instance, at level 5 the max CP cap you can power up to might be 100, but at level 20 that same Pokémon's max CP cap might be 600.