Recently I had to refactor some of the predicate-generating code for time series to fix SERVER-73641. In the new code I wrote I made use of MatchExpressionVisitor and MatchExpressionWalker to make changes to match expressions. This code is not yet merged but I hope that it will be soon:

https://github.com/10gen/mongo/blob/22331af6fda05f58f02a48ced34c3d9486b8b9ca/src/mongo/db/query/timeseries/time_predicate_augmenter.cpp#L138-L149

Now this new code does not match the style of the old code, which could be confusing. We should update the old code in bucket_level_comparison_predicate_generator.cpp to use walkers and visitors, since that is (my opinion) easier to understand.