Hi Marshall, I recommend in these types of scenarios that you try it out in Preview to see what exactly happens with your data when you make changes to existing fields. In this case, you won't lose any historical data by changing a field from multi-select to single-select. Any old objects that still have multiple values selected will display the selected values delimited by a comma. Users can still edit those values, but will only be able to select a single value.
You can also then be able to group by that field. The Grouping will treat objects that have multiple values as a unique value based on its combination of selections. For example: Option 1 Option 2 Option 3 Option 2, Option 3 Option 1, Option 2 Option 1, Option 3 Overall I've found it to be a pretty seamless transition and am really happy to be able to change the behavior/display of existing fields. Hope that helps! William English T-Mobile
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