Nothing formal has been announced for the A321 apart from training and technical positions. The first 3 won’t even be here til mid 2025 so far off announcing solid plans yet.
Eventually however the plan will be for all bases to be fully transferred to the 321 in 10 years, but there’ll be some 737 bases remaining too as that aircraft will keep operating for 10 years. They may run 2 aircraft per base for a while and then eventually fully transfer all flying to the 321 but the exact timeframe for that hasn’t been decided.