When moving through feeding stages, you can choose between:     
1) Normal Progression (like vanilla Skyrim); stronger when starving
2) Reverse Progression; stronger when fully fed
            
To advance through vampiric ranks, you can choose:   
1) Normal Ranks (steeper level and necks bitten requirements)
2) Easier Ranks (more relaxed level and necks bitten requirements)
3) Days as a Vampire (only your vampiric age determines your rank)

NOTE:    You can remove the level requirement in the MCM if you wish.  If you select "Days as a Vampire" progression, the default is rank advancement every 40 game days, but this can be customized in the MCM.

            I STRONGLY recommend against flipping back and forth between rank progressions ... if you do want to
            change, use the MCM to change your progression and then reset vampirism. 
            This will never take away your necks bitten, or days as a Vampire, but it will adjust you to the proper rank.