Question about designing encounters

So I'm designing the boss fight for the adventure my pcs are going on, and the fight will be in 2 stages. The first, the party will fight a mage and some cultists, after killing everyone they will exit combat. The 2nd part is as they approach the body of the mage, it transforms into a modified bone devil and they enter combat again. My question is how do I treat the CR of this fight? Do I group the 2 stages together and consider it 1 encounter or can I consider them separate encounters with separate CRs?

Thanks!