The leases of new planes are killers. Check accurately the expenses of your new planes, compare them to the older ones, and do the maths. The "best" planes, as the A321, are so much in demand that their prices go insanely high. And it makes them less interesting.
In previous GW3, I replaced my CRJs by inferior A148...and this move allowed me to grow much quicker and to make more money. Best planes are not always a good choice. Of course, if you're swimming in cash, and you can afford to buy brand new "best" planes(I'm currently rich & doing that with 737MAX ans Ejets), then of course proceed. But, if you have to lease, then think twice. Old leased crap may be actually cheaper, depending on how costly is fuel, and how costly are the other planes. It's a complex maths to do, and as plane prices are dynamic, it's really not an easy choice. But don't think that "the best is always better". It often is. But sometimes not. I learned it the hard way with 737-700ER. I love those babies, but don't loan them. it's suicide(fortunately, I had taken only 3 of them, just to see. Once bought, they were very productive).
There is also the communality effect, but in your case, you are too small for it being significant. Unless you have 4 fleet types? If you are still at 3 or less fleet types, communality costs should not be a big deal.