immutable to mutable bucket

immutable to mutable bucket

We recently had to add an additional variable to be used for our solution. The initial setup is using an immutable bucket with a retention of 1 year. However, with the additional variable we need to change strategy. The 2 variables need to  have different retention periods.
  1. Is our only option to change back from immutable to mutable and so manage the retentions from an analysis. 
    1. If so, how to transfer from immutable to mutable without emptying the buckets.
  2. Or can we exclude variables from retention. 
Thanks,
Vincent