Migrate from daily indices to ILM rollover with same index pattern?

We have a bunch of daily indices matched by the index pattern lambda-* e.g. lambda-2020-02-02. We want to start using a size-based ilm rollover policy but we don't want to break our existing visualisations etc.

Is it possible for us to set up an index alias lambda and reindex the old date-based indexes onto the new alias?

Our plan is to modify the existing template with the new policy. What will happen?

Our expectation is that we can continue to write date-based indexes so long as they don't exceed the max age/size of the ilm policy. And then that'll give us the ability to reindex in the background and modify ingestion as a final step.

Our current plan:
Day 1: we ingest into lambda-2020-01-01
Day 2: we update the lambda-* template with the new policy and ingest into lambda-2020-01-02
At this point, lambda-2020-01-01 has no ILM policy, lambda-2020-01-02 has the new policy attached. We also bootstrap lambda with lambda-000001
Day 3: we ingest into lambda-2020-01-03 and also reindex lambda-2020-01-01 and lambda-2020-01-02 onto lambda (which would write into the bootstrapped lambda-00001)
Day 4: we switch ingestion to lambda and then reindex lambda-2020-01-03 onto lambda
Day 5: ingestion continues to lambda and we are finished migrating

Are we missing any subtleties that would stop this from working?

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